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  25.     <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav>  <ul class="sidebar-links"><li><a href="/blog/pages/0796ba76b4b55368/" class="sidebar-link">基础</a></li><li><a href="/blog/pages/74d2ab3fbfeaaa68/" aria-current="page" class="active sidebar-link">内置对象</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#一、object对象" class="sidebar-link">一、Object对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、概述" class="sidebar-link">1、概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、object-当作工具方法-不通过new创建" class="sidebar-link">2、Object() 当作工具方法（不通过new创建）</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、object构造函数-通过new创建" class="sidebar-link">3、Object构造函数（通过new创建）</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、object-的静态方法" class="sidebar-link">4、Object 的静态方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-keys" class="sidebar-link">Object.keys()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-getownpropertynames" class="sidebar-link">Object.getOwnPropertyNames()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object的其他静态方法" class="sidebar-link">Object的其他静态方法</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1-对象属性模型的相关方法" class="sidebar-link">（1）对象属性模型的相关方法</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-控制对象状态的方法" class="sidebar-link">（2）控制对象状态的方法</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-原型链相关方法" class="sidebar-link">（3）原型链相关方法</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5、object-的实例方法" class="sidebar-link">5、Object 的实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-prototype-valueof" class="sidebar-link">Object.prototype.valueOf()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-prototype-tostring" class="sidebar-link">Object.prototype.toString()</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#tostring-的应用-判断数据类型" class="sidebar-link">toString() 的应用：判断数据类型</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-prototype-tolocalestring" class="sidebar-link">Object.prototype.toLocaleString()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-prototype-hasownproperty" class="sidebar-link">Object.prototype.hasOwnProperty()</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#二、属性描述对象" class="sidebar-link">二、属性描述对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、概述-2" class="sidebar-link">1、概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、object-getownpropertydescriptor" class="sidebar-link">2、Object.getOwnPropertyDescriptor()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、object-getownpropertynames" class="sidebar-link">3、Object.getOwnPropertyNames()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、object-defineproperty-object-defineproperties" class="sidebar-link">4、Object.defineProperty()，Object.defineProperties()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5、object-prototype-propertyisenumerable" class="sidebar-link">5、Object.prototype.propertyIsEnumerable()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_6、元属性" class="sidebar-link">6、元属性</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#value目标属性的值" class="sidebar-link">value目标属性的值</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#writable可写性" class="sidebar-link">writable可写性</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#enumerable可遍历性" class="sidebar-link">enumerable可遍历性</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#configurable可配置性" class="sidebar-link">configurable可配置性</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_7、存取器-get和set" class="sidebar-link">7、存取器（get和set）</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_8、对象的拷贝" class="sidebar-link">8、对象的拷贝</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_9、控制对象读写状态" class="sidebar-link">9、控制对象读写状态</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-preventextensions-防止新增" class="sidebar-link">Object.preventExtensions() 防止新增</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-isextensible-是可新增的" class="sidebar-link">Object.isExtensible() 是可新增的？</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-seal-密封" class="sidebar-link">Object.seal() 密封</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-issealed-是密封的" class="sidebar-link">Object.isSealed() 是密封的？</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-freeze-冻结" class="sidebar-link">Object.freeze() 冻结</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#object-isfrozen-是冻结的" class="sidebar-link">Object.isFrozen() 是冻结的？</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#局限性" class="sidebar-link">局限性</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#三、array对象" class="sidebar-link">三、Array对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、array构造函数" class="sidebar-link">1、Array构造函数</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、静态方法" class="sidebar-link">2、静态方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#array-isarray" class="sidebar-link">Array.isArray()</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、实例方法" class="sidebar-link">3、实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#valueof-原数组-tostring-转字符串" class="sidebar-link">valueOf() 原数组，toString() 转字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#push-末尾添加-pop-删除末尾" class="sidebar-link">push() 末尾添加，pop() 删除末尾</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#unshift-首位添加-shift-删除首位" class="sidebar-link">unshift() 首位添加, shift() 删除首位</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#join-连接每个成员-转字符串" class="sidebar-link">join() 连接每个成员-转字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#concat-合并数组" class="sidebar-link">concat() 合并数组</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#reverse-反转" class="sidebar-link">reverse() 反转</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#slice-部分-提取部分成员" class="sidebar-link">slice() 部分-提取部分成员</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#应用-类数组转数组" class="sidebar-link">应用-类数组转数组</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#splice-从第几个开始-删除几个-并添加成员-返回被删除成员。" class="sidebar-link">splice()  从第几个开始，删除几个，并添加成员，返回被删除成员。</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#应用-拆分数组" class="sidebar-link">应用-拆分数组</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#sort-排序" class="sidebar-link">sort() 排序</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#应用-自定义排序" class="sidebar-link">应用-自定义排序</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#map-映射-每个成员-并返回" class="sidebar-link">map() 映射 每个成员 并返回</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#foreach-循环每个成员" class="sidebar-link">forEach() 循环每个成员</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#filter-过滤" class="sidebar-link">filter() 过滤</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#some-某个-every-每个" class="sidebar-link">some()某个，every()每个</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#reduce-累计器-reduceright-反向累计器" class="sidebar-link">reduce() 累计器，reduceRight() 反向累计器</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#应用-找出字符最长的数组成员" class="sidebar-link">应用-找出字符最长的数组成员</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#应用-按属性对object分类" class="sidebar-link">应用-按属性对object分类</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#indexof-搜索成员第一个出现的位置-lastindexof-搜索成员最后出现的位置" class="sidebar-link">indexOf() 搜索成员第一个出现的位置，lastIndexOf() 搜索成员最后出现的位置</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#链式使用" class="sidebar-link">链式使用</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#四、包装对象" class="sidebar-link">四、包装对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、定义" class="sidebar-link">1、定义</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、实例方法" class="sidebar-link">2、实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-1-valueof" class="sidebar-link">2.1 valueOf()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-2-tostring" class="sidebar-link">2.2 toString()</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、原始类型与实例对象的自动转换" class="sidebar-link">3、原始类型与实例对象的自动转换</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、自定义方法" class="sidebar-link">4、自定义方法</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#五、boolean对象" class="sidebar-link">五、Boolean对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、概述-3" class="sidebar-link">1、概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、boolean-函数的类型转换作用" class="sidebar-link">2、Boolean 函数的类型转换作用</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#六、number对象" class="sidebar-link">六、Number对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、概述-4" class="sidebar-link">1、概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、静态属性" class="sidebar-link">2、静态属性</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、实例方法-2" class="sidebar-link">3、实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-1-number-prototype-tostring-可转成多种进制字符串" class="sidebar-link">3.1 Number.prototype.toString() 可转成多种进制字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-2-number-prototype-tofixed-转成保留指定小数位数的字符串" class="sidebar-link">3.2 Number.prototype.toFixed() 转成保留指定小数位数的字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-3-number-prototype-toexponential-转成科学计数法形式字符串" class="sidebar-link">3.3 Number.prototype.toExponential() 转成科学计数法形式字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-4-number-prototype-toprecision-转为指定位数的有效数字" class="sidebar-link">3.4 Number.prototype.toPrecision()转为指定位数的有效数字</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-5-number-prototype-tolocalestring" class="sidebar-link">4.5 Number.prototype.toLocaleString()</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#应用-添加千位分隔符" class="sidebar-link">应用-添加千位分隔符</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、自定义方法-2" class="sidebar-link">4、自定义方法</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#七、string对象" class="sidebar-link">七、String对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、概述-5" class="sidebar-link">1、概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、静态方法-2" class="sidebar-link">2、静态方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#string-fromcharcode" class="sidebar-link">String.fromCharCode()</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、实例属性" class="sidebar-link">3、实例属性</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#string-prototype-length" class="sidebar-link">String.prototype.length</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、实例方法" class="sidebar-link">4、实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-1-string-prototype-charat-返回指定位置的字符" class="sidebar-link">4.1 String.prototype.charAt() 返回指定位置的字符</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-2-string-prototype-charcodeat-返回字符串指定位置的-unicode-码点-十进制表示" class="sidebar-link">4.2 String.prototype.charCodeAt()返回字符串指定位置的 Unicode 码点（十进制表示）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-3-string-prototype-concat-合并字符串-返回新字符串-不改变原字符串" class="sidebar-link">4.3 String.prototype.concat() 合并字符串，返回新字符串，不改变原字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-4-string-prototype-slice-取出部分子字符串-不改变原字符串" class="sidebar-link">4.4 String.prototype.slice() 取出部分子字符串，不改变原字符串</a></li><li class="sidebar-sub-header level5"><a href="/blog/pages/74d2ab3fbfeaaa68/#负数表示倒数" class="sidebar-link">负数表示倒数</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-5-string-prototype-substring-和slice类似" class="sidebar-link">4.5 String.prototype.substring() 和slice类似</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#小总结-substring-和-slice-的区别-有两个。" class="sidebar-link">小总结：substring() 和 slice() 的区别，有两个。</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-6-string-prototype-substr-和-slice、substring类似-区别是参数二表示长度" class="sidebar-link">4.6 String.prototype.substr() 和 slice、substring类似，区别是参数二表示长度</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-7-string-prototype-indexof-搜索子串第一个出现的位置-string-prototype-lastindexof-搜索子串最后出现的位置" class="sidebar-link">4.7 String.prototype.indexOf() 搜索子串第一个出现的位置，String.prototype.lastIndexOf() 搜索子串最后出现的位置</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-8-string-prototype-trim-修剪-两端空格-不改变原字符串" class="sidebar-link">4.8 String.prototype.trim()  ''修剪'‘两端空格，不改变原字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-9-string-prototype-tolowercase-全转小写-string-prototype-touppercase-全转大写-都不改变原字符串" class="sidebar-link">4.9 String.prototype.toLowerCase() 全转小写，String.prototype.toUpperCase() 全转大写，都不改变原字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-10-string-prototype-match-匹配子串-返回数组-数组带index、input属性" class="sidebar-link">4.10 String.prototype.match() 匹配子串，返回数组（数组带index、input属性）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-11-string-prototype-search-搜索子串第一次出现的位置-没有则返-1-string-prototype-replace-替换子串-参数二替换参数一-不改变原字符串" class="sidebar-link">4.11 String.prototype.search() 搜索子串第一次出现的位置，没有则返-1，String.prototype.replace() 替换子串，参数二替换参数一，不改变原字符串</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-12-string-prototype-split-按给定规则分割字符串-返回数组" class="sidebar-link">4.12 String.prototype.split() 按给定规则分割字符串，返回数组</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-13-string-prototype-localecompare-比较两个字符串-返回整数。" class="sidebar-link">4.13 String.prototype.localeCompare() 比较两个字符串，返回整数。</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#八、math对象" class="sidebar-link">八、Math对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、静态属性" class="sidebar-link">1、静态属性</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、静态方法-3" class="sidebar-link">2、静态方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-1-math-abs-绝对值" class="sidebar-link">2.1 Math.abs() 绝对值</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-2-math-max-最大值-math-min-最-小值" class="sidebar-link">2.2 Math.max() 最大值，Math.min()最 小值</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-3-math-floor-向下取整-math-ceil-向上取整" class="sidebar-link">2.3 Math.floor() 向下取整，Math.ceil() 向上取整</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-4-math-round-四舍五入" class="sidebar-link">2.4 Math.round() 四舍五入</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-5-math-pow-指数运算-次方运算" class="sidebar-link">2.5 Math.pow() 指数运算 （次方运算）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-6-math-sqrt-平方根" class="sidebar-link">2.6 Math.sqrt() 平方根</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-7-math-log-自然对数" class="sidebar-link">2.7 Math.log() 自然对数</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-8-math-exp-e的指数" class="sidebar-link">2.8 Math.exp() e的指数</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-9-math-random-0到1的随机数-可能等于0-但一定小于1" class="sidebar-link">2.9 Math.random() 0到1的随机数（可能等于0，但一定小于1）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-10-三角函数方法" class="sidebar-link">2.10 三角函数方法</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#九、date对象" class="sidebar-link">九、Date对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、普通函数的用法" class="sidebar-link">1、普通函数的用法</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、构造函数的用法" class="sidebar-link">2、构造函数的用法</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、日期的运算" class="sidebar-link">3、日期的运算</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、静态方法" class="sidebar-link">4、静态方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-1-date-now-当前时间戳" class="sidebar-link">4.1 Date.now() 当前时间戳</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-2-date-parse-解析日期字符串-返回该时间点的时间戳" class="sidebar-link">4.2 Date.parse() 解析日期字符串，返回该时间点的时间戳</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-3-date-utc-以年、月、日等变量作为参数-返回该时间点的时间戳" class="sidebar-link">4.3 Date.UTC() 以年、月、日等变量作为参数，返回该时间点的时间戳</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5、实例方法" class="sidebar-link">5、实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-1-date-prototype-valueof-返回时间戳-等同于gettime-。-减法运算默认调用" class="sidebar-link">5.1 Date.prototype.valueOf() 返回时间戳，等同于getTime()。（减法运算默认调用）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-2-to-类方法-返回字符串相关" class="sidebar-link">5.2 to 类方法（返回字符串相关）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1-date-prototype-tostring-返回一个完整的日期字符串-直接读取实例默认调用此方法" class="sidebar-link">（1）Date.prototype.toString() 返回一个完整的日期字符串（直接读取实例默认调用此方法）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-date-prototype-toutcstring-返回对应的utc时间-世界标准时-比北京时间晚8小时" class="sidebar-link">（2）Date.prototype.toUTCString() 返回对应的UTC时间（世界标准时），比北京时间晚8小时</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-date-prototype-toisostring-返回对应时间的-iso8601-写法-utc时间" class="sidebar-link">（3）Date.prototype.toISOString() 返回对应时间的 ISO8601 写法（UTC时间）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-date-prototype-tojson-返回符合json格式字符串-与toisostring方法的返回结果完全相同。" class="sidebar-link">（4）Date.prototype.toJSON() 返回符合JSON格式字符串，与toISOString方法的返回结果完全相同。</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-date-prototype-todatestring-返回日期字符串-不含时分秒" class="sidebar-link">（5）Date.prototype.toDateString() 返回日期字符串（不含时分秒）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_6-date-prototype-totimestring-返回时间字符串-不含年月日" class="sidebar-link">（6）Date.prototype.toTimeString() 返回时间字符串（不含年月日）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_7-本地时间" class="sidebar-link">（7）本地时间</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-3-get类-获取年、月、日、时、分、秒等" class="sidebar-link">5.3 get类 （获取年、月、日、时、分、秒等）</a></li><li class="sidebar-sub-header level6"><a href="/blog/pages/74d2ab3fbfeaaa68/#例子-计算本年度还剩下多少天" class="sidebar-link">例子：计算本年度还剩下多少天</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-4-set-类方法" class="sidebar-link">5.4 set 类方法</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#十、regexp-对象" class="sidebar-link">十、RegExp 对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、概述-6" class="sidebar-link">1、概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、实例属性" class="sidebar-link">2、实例属性</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、实例方法-3" class="sidebar-link">3、实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-1-regexp-prototype-test-当前模式是否匹配参数字符串-返回布尔值" class="sidebar-link">3.1 RegExp.prototype.test() 当前模式是否匹配参数字符串，返回布尔值</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-2-regexp-prototype-exec-返回匹配结果-如匹配则返回一个数组-成员是匹配到的子字符串-否则返回null" class="sidebar-link">3.2 RegExp.prototype.exec() 返回匹配结果，如匹配则返回一个数组，成员是匹配到的子字符串，否则返回null</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、字符串的实例方法" class="sidebar-link">4、字符串的实例方法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-1-string-prototype-match-匹配-返回匹配结果数组或null" class="sidebar-link">4.1 String.prototype.match() 匹配，返回匹配结果数组或null</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-2-string-prototype-search-返回第一个满足匹配条件的位置-不满足则返-1" class="sidebar-link">4.2 String.prototype.search() 返回第一个满足匹配条件的位置，不满足则返-1</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-3-string-prototype-replace-替换匹配的值-参数一是子串或正则-参数二是替换内容" class="sidebar-link">4.3 String.prototype.replace() 替换匹配的值，参数一是子串或正则，参数二是替换内容</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-4-string-prototype-split-按给定规则分割字符串-返回数组" class="sidebar-link">4.4 String.prototype.split() 按给定规则分割字符串，返回数组</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5、匹配规则" class="sidebar-link">5、匹配规则</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-1-字面量字符和元字符" class="sidebar-link">5.1 字面量字符和元字符</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1-点字符-类似通配" class="sidebar-link">（1）点字符（.)   类似通配</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-位置字符-开始位置和结束位置" class="sidebar-link">（2）位置字符(^) ($)  开始位置和结束位置</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-选择符-表示-或关系" class="sidebar-link">（3）选择符（|） 表示''或关系''</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-2-转义符-反斜杠" class="sidebar-link">5.2 转义符（\ 反斜杠）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-3-特殊字符" class="sidebar-link">5.3 特殊字符</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-4-字符类" class="sidebar-link">5.4 字符类</a></li><li class="sidebar-sub-header level5"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1-脱字符-排除" class="sidebar-link">（1）脱字符（^  排除）</a></li><li class="sidebar-sub-header level5"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-连字符-字符连续范围" class="sidebar-link">（2）连字符（- 字符连续范围）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-5-预定义模式-常见模式简写-d-d-w-w-s-s-b-b" class="sidebar-link">5.5 预定义模式 (常见模式简写\d\D\w\W\s\S\b\B)</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-6-重复类-n-n-n-m" class="sidebar-link">5.6 重复类 （ {n} {n,} {n,m} ）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-7-量词符" class="sidebar-link">5.7 量词符（?）（*）（+）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-8-贪婪模式-对量词符可使用非贪婪模式" class="sidebar-link">5.8 贪婪模式（对量词符可使用非贪婪模式）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-9-修饰符" class="sidebar-link">5.9 修饰符</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1-g-修饰符-全局匹配" class="sidebar-link">（1）g 修饰符 （全局匹配）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-i-修饰符-不区分大小写" class="sidebar-link">（2）i 修饰符 （不区分大小写）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-m-修饰符-让-和-识别换行符" class="sidebar-link">（3）m 修饰符（让^和$识别换行符）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_5-10-组匹配" class="sidebar-link">5.10 组匹配</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1-概述-圆括号分组" class="sidebar-link">（1）概述 （圆括号分组）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2-非捕获组" class="sidebar-link">（2）非捕获组</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-先行断言" class="sidebar-link">（3）先行断言</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4-先行否定断言" class="sidebar-link">（4）先行否定断言</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#十一、json对象" class="sidebar-link">十一、JSON对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_1、json-格式" class="sidebar-link">1、JSON 格式</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_2、json-对象" class="sidebar-link">2、JSON 对象</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3、json-stringify-转为json字符串" class="sidebar-link">3、JSON.stringify() 转为JSON字符串</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-1-基本用法" class="sidebar-link">3.1 基本用法</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-2-第二个参数-一个数组-对象属性白名单-或一个函数-改写返回值" class="sidebar-link">3.2 第二个参数 （一个数组，对象属性白名单；或一个函数，改写返回值）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-3-第三个参数-增加返回json字符串的可读性-json格式" class="sidebar-link">3.3 第三个参数 （增加返回JSON字符串的可读性；json格式）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/74d2ab3fbfeaaa68/#_3-4-参数对象的-tojson-方法-对象自定义tojson方法-转字符串只取该方法返回值" class="sidebar-link">3.4 参数对象的 toJSON 方法（对象自定义toJSON方法，转字符串只取该方法返回值）</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#_4、json-parse" class="sidebar-link">4、JSON.parse()</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/74d2ab3fbfeaaa68/#文档" class="sidebar-link">文档</a></li></ul></li><li><a href="/blog/pages/659b5af5e2e704e0/" class="sidebar-link">面向对象</a></li><li><a href="/blog/pages/d61b1cb4cdac1f63/" class="sidebar-link">异步操作</a></li><li><a href="/blog/pages/7d961b8030c6099e/" class="sidebar-link">DOM</a></li><li><a href="/blog/pages/10b2761db5a8e089/" class="sidebar-link">事件</a></li><li><a href="/blog/pages/bab4930124ad2c10/" class="sidebar-link">浏览器模型</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/javascript/#《JavaScript教程》笔记" data-v-06970110>《JavaScript教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="https://github.com/heBody" target="_blank" title="作者" class="beLink" data-v-06970110>heBody</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-01-12</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">内置对象<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="内置对象篇"><a href="#内置对象篇" class="header-anchor">#</a> 内置对象篇</h1> <h2 id="一、object对象"><a href="#一、object对象" class="header-anchor">#</a> 一、Object对象</h2> <h2 id="_1、概述"><a href="#_1、概述" class="header-anchor">#</a> 1、概述</h2> <p>JavaScript 原生提供<code>Object</code>对象（注意起首的<code>O</code>是大写），本章介绍该对象原生的各种方法。</p> <p>JavaScript 的所有其他对象都继承自<code>Object</code>对象，即那些对象都是<code>Object</code>的实例。</p> <p><code>Object</code>对象的原生方法分成两类：<code>Object</code>本身的方法与<code>Object</code>的实例方法。</p> <p><strong>（1）Object对象本身的方法</strong></p> <p>所谓“本身的方法”就是直接定义在<code>Object</code>对象的方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Object<span class="token punctuation">.</span><span class="token function-variable function">print</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">o</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    26. 

  26. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>print</code>方法就是直接定义在<code>Object</code>对象上。</p> <p><strong>（2）Object的实例方法</strong></p> <p>所谓实例方法就是定义在<code>Object</code>原型对象<code>Object.prototype</code>上的方法。它可以被<code>Object</code>实例直接使用。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">print</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    27. 

  27.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    28. 

  28. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    29. 

  29. 

  30. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    31. 

  31. obj<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Object</span>
    32. 

  32. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>Object.prototype</code>定义了一个<code>print</code>方法，然后生成一个<code>Object</code>的实例<code>obj</code>。<code>obj</code>直接继承了<code>Object.prototype</code>的属性和方法，可以直接使用<code>obj.print</code>调用<code>print</code>方法。也就是说，<code>obj</code>对象的<code>print</code>方法实质上就是调用<code>Object.prototype.print</code>方法。</p> <p>关于原型对象<code>object.prototype</code>的详细解释，参见《面向对象编程》章节。这里只要知道，凡是定义在<code>Object.prototype</code>对象上面的属性和方法，将被所有实例对象共享就可以了。</p> <p>以下先介绍<code>Object</code>作为函数的用法，然后再介绍<code>Object</code>对象的原生方法，分成对象自身的方法（又称为“静态方法”）和实例方法两部分。</p> <h2 id="_2、object-当作工具方法-不通过new创建"><a href="#_2、object-当作工具方法-不通过new创建" class="header-anchor">#</a> 2、Object() 当作工具方法（不通过new创建）</h2> <p><code>Object()</code><strong>本身是一个函数</strong>，可以当作工具方法使用，将任意值转为对象。这个方法常用于保证某个值一定是对象。</p> <p>如果参数为空（或者为<code>undefined</code>和<code>null</code>），<code>Object()</code>返回一个空对象。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// obj =&gt; {}</span>
    33. 

  33. <span class="token comment">// 等同于</span>
    34. 

  34. obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span>
    35. 

  35. obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span>
    36. 

  36. obj <span class="token keyword">instanceof</span> <span class="token class-name">Object</span> <span class="token comment">// true</span>
    37. 

  37. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码的含义，是将<code>undefined</code>和<code>null</code>转为对象，结果得到了一个空对象<code>obj</code>。</p> <p><code>instanceof</code>运算符用来验证，一个对象是否为指定的构造函数的实例。<code>obj instanceof Object</code>返回<code>true</code>，就表示<code>obj</code>对象是<code>Object</code>的实例。</p> <p>如果参数是原始类型的值，<code>Object</code>方法将其转为对应的包装对象的实例（参见《原始类型的包装对象》一章）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 包装对象实例</span>
    38. 

  38. 

  39. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// obj =&gt; Number{1}</span>
    40. 

  40. obj <span class="token keyword">instanceof</span> <span class="token class-name">Object</span> <span class="token comment">// true     in s tan ce of 判断对象类型</span>
    41. 

  41. obj <span class="token keyword">instanceof</span> <span class="token class-name">Number</span> <span class="token comment">// true</span>
    42. 

  42. 

  43. 

  44. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// obj =&gt; String{‘foo’}</span>
    45. 

  45. obj <span class="token keyword">instanceof</span> <span class="token class-name">Object</span> <span class="token comment">// true</span>
    46. 

  46. obj <span class="token keyword">instanceof</span> <span class="token class-name">String</span> <span class="token comment">// true</span>
    47. 

  47. 

  48. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// obj =&gt; Boolean{‘foo’}</span>
    49. 

  49. obj <span class="token keyword">instanceof</span> <span class="token class-name">Object</span> <span class="token comment">// true</span>
    50. 

  50. obj <span class="token keyword">instanceof</span> <span class="token class-name">Boolean</span> <span class="token comment">// true</span>
    51. 

  51. 

  52. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，<code>Object</code>函数的参数是各种原始类型的值，转换成对象就是原始类型值对应的包装对象。</p> <p>如果<code>Object</code>方法的参数是一个对象，它总是返回该对象，即不用转换。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
    53. 

  53. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span> <span class="token comment">// obj =&gt; []</span>
    54. 

  54. obj <span class="token operator">===</span> arr <span class="token comment">// true</span>
    55. 

  55. 

  56. 

  57. <span class="token keyword">var</span> value <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    58. 

  58. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token comment">// obj =&gt; {}</span>
    59. 

  59. obj <span class="token operator">===</span> value <span class="token comment">// true</span>
    60. 

  60. 

  61. 

  62. <span class="token keyword">var</span> <span class="token function-variable function">fn</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    63. 

  63. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span>fn<span class="token punctuation">)</span> <span class="token comment">// obj =&gt; function(){}</span>
    64. 

  64. obj <span class="token operator">===</span> fn <span class="token comment">// true</span>
    65. 

  65. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>利用这一点，可以写一个<strong>判断变量是否为对象</strong>的函数。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>function isObject(value){
    66. 

  66.     return value === Object(value)
    67. 

  67. }
    68. 

  68. 

  69. isObject([]) // true
    70. 

  70. isObject({}) // true
    71. 

  71. isObject('123') // false
    72. 

  72. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h2 id="_3、object构造函数-通过new创建"><a href="#_3、object构造函数-通过new创建" class="header-anchor">#</a> 3、Object构造函数（通过new创建）</h2> <p><code>Object</code>构造函数的首要用途，是直接通过它来<strong>生成新对象</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 等同于 var obj = {}</span>
    73. 

  73. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><blockquote><p>注意，通过<code>var obj = new Object()</code>的写法生成新对象，与字面量的写法<code>var obj = {}</code>是等价的。或者说，后者只是前者的一种简便写法。</p></blockquote> <p><code>Object</code>构造函数的用法与工具方法很相似，几乎一模一样。使用时，可以接受一个参数，如果该参数是一个对象，则直接返回这个对象；如果是一个原始类型的值，则返回该值对应的包装对象（详见《包装对象》一章）。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var o1 = {a: 1};
    74. 

  74. var o2 = new Object(o1);
    75. 

  75. o1 === o2 // true
    76. 

  76. 

  77. var obj = new Object(123);
    78. 

  78. obj instanceof Number // true
    79. 

  79. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>虽然用法相似，但是<code>Object(value)</code>与<code>new Object(value)</code>两者的语义是不同的，<code>Object(value)</code>表示将<code>value</code>转成一个对象，<code>new Object(value)</code>则表示新生成一个对象，它的值是<code>value</code>。</p> <h2 id="_4、object-的静态方法"><a href="#_4、object-的静态方法" class="header-anchor">#</a> 4、Object 的静态方法</h2> <p>所谓“静态方法”，是指<strong>部署在<code>Object</code>对象自身</strong>的方法。</p> <h3 id="object-keys"><a href="#object-keys" class="header-anchor">#</a> Object.keys()</h3> <p><code>Object.keys()</code>方法和<code>Object.getOwnPropertyNames()</code>方法都用来遍历对象的属性。</p> <p><code>Object.keys()</code>方法的参数是一个对象，返回一个数组。该数组的成员都是该对象自身的（而不是继承的）所有<strong>属性名</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    80. 

  80.     <span class="token literal-property property">p1</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
    81. 

  81.     <span class="token literal-property property">p2</span><span class="token operator">:</span> <span class="token number">456</span>
    82. 

  82. <span class="token punctuation">}</span>
    83. 

  83. 

  84. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// ['p1','p2']  注意是属性名</span>
    85. 

  85. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h3 id="object-getownpropertynames"><a href="#object-getownpropertynames" class="header-anchor">#</a> Object.getOwnPropertyNames()</h3> <p><code>Object.getOwnPropertyNames</code>方法与<code>Object.keys</code>类似，也是接受一个对象作为参数，返回一个数组，包含了该对象自身的所有属性名。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    86. 

  86.     <span class="token literal-property property">p1</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
    87. 

  87.     <span class="token literal-property property">p2</span><span class="token operator">:</span> <span class="token number">456</span>
    88. 

  88. <span class="token punctuation">}</span>
    89. 

  89. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// ['p1','p2']</span>
    90. 

  90. <span class="token comment">// getOwnPropertyNames 中文含义：获取自己的属性名</span>
    91. 

  91. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>对于一般的对象来说，<code>Object.keys()</code>和<code>Object.getOwnPropertyNames()</code>返回的结果是一样的。只有涉及不可枚举属性时，才会有不一样的结果。<code>Object.keys</code>方法只返回可枚举的属性（详见《对象属性的描述对象》一章），<code>Object.getOwnPropertyNames</code>方法还返回<strong>不可枚举的属性名</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'hello'</span><span class="token punctuation">,</span><span class="token string">'world'</span><span class="token punctuation">]</span>
    92. 

  92. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// ['0',‘1’]  如果对象是数组时返回索引字符串</span>
    93. 

  93. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// [‘0’,‘1’,'length']</span>
    94. 

  94. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，数组的<code>length</code>属性是<strong>不可枚举的属性</strong>，所以只出现在<code>Object.getOwnPropertyNames</code>方法的返回结果中。</p> <p>由于 JavaScript 没有提供计算对象属性个数的方法，所以可以用这两个方法代替。</p> <p><strong>获取对象属性个数长度</strong></p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var obj = {
    95. 

  95.   p1: 123,
    96. 

  96.   p2: 456
    97. 

  97. };
    98. 

  98. 

  99. Object.keys(obj).length // 2
    100. 

  100. Object.getOwnPropertyNames(obj).length // 2
    101. 

  101. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>一般情况下，几乎总是使用<code>Object.keys</code>方法，遍历对象的属性。</p> <h3 id="object的其他静态方法"><a href="#object的其他静态方法" class="header-anchor">#</a> Object的其他静态方法</h3> <p>除了上面提到的两个方法，<code>Object</code>还有不少其他静态方法，将在后文逐一详细介绍。</p> <h4 id="_1-对象属性模型的相关方法"><a href="#_1-对象属性模型的相关方法" class="header-anchor">#</a> （1）对象属性模型的相关方法</h4> <ul><li><code>Object.getOwnPropertyDescriptor()</code>：获取某个属性的描述对象。</li> <li><code>Object.defineProperty()</code>：通过描述对象，定义某个属性。</li> <li><code>Object.defineProperties()</code>：通过描述对象，定义多个属性。</li></ul> <h4 id="_2-控制对象状态的方法"><a href="#_2-控制对象状态的方法" class="header-anchor">#</a> （2）控制对象状态的方法</h4> <ul><li><code>Object.preventExtensions()</code>：防止对象扩展。</li> <li><code>Object.isExtensible()</code>：判断对象是否可扩展。</li> <li><code>Object.seal()</code>：禁止对象配置。</li> <li><code>Object.isSealed()</code>：判断一个对象是否可配置。</li> <li><code>Object.freeze()</code>：冻结一个对象。</li> <li><code>Object.isFrozen()</code>：判断一个对象是否被冻结。</li></ul> <h4 id="_3-原型链相关方法"><a href="#_3-原型链相关方法" class="header-anchor">#</a> （3）原型链相关方法</h4> <ul><li><code>Object.create()</code>：该方法可以指定原型对象和属性，返回一个新的对象。</li> <li><code>Object.getPrototypeOf()</code>：获取对象的<code>Prototype</code>对象。</li></ul> <p>、</p> <p>有部分方法为列入此文档，更多方法详情：<a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object" target="_blank" rel="noopener noreferrer">https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <h2 id="_5、object-的实例方法"><a href="#_5、object-的实例方法" class="header-anchor">#</a> 5、Object 的实例方法</h2> <p>除了静态方法，还有不少方法定义在<code>Object.prototype</code>对象。它们称为实例方法，所有<code>Object</code>的实例对象都继承了这些方法。</p> <p><code>Object</code>实例对象的方法，主要有以下六个。</p> <ul><li><code>Object.prototype.valueOf()</code>：返回当前对象对应的值。</li> <li><code>Object.prototype.toString()</code>：返回当前对象对应的字符串形式。</li> <li><code>Object.prototype.toLocaleString()</code>：返回当前对象对应的本地字符串形式。</li> <li><code>Object.prototype.hasOwnProperty()</code>：判断某个属性是否为当前对象自身的属性，还是继承自原型对象的属性。</li> <li><code>Object.prototype.isPrototypeOf()</code>：判断当前对象是否为另一个对象的原型。</li> <li><code>Object.prototype.propertyIsEnumerable()</code>：判断某个属性是否可枚举。</li></ul> <p>本节介绍前四个方法，另外两个方法将在后文相关章节介绍。</p> <h3 id="object-prototype-valueof"><a href="#object-prototype-valueof" class="header-anchor">#</a> Object.prototype.valueOf()</h3> <p><code>valueOf</code>方法的作用是返回一个对象的“值”，默认情况下<strong>返回对象本身</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    102. 

  102. obj<span class="token punctuation">.</span><span class="token function">valueof</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">===</span> obj <span class="token comment">//true</span>
    103. 

  103. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码比较<code>obj.valueOf()</code>与<code>obj</code>本身，两者是一样的。</p> <p><code>valueOf</code>方法的主要用途是，JavaScript <strong>自动类型转换</strong>时会默认调用这个方法（详见《数据类型转换》一章）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token function">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    104. 

  104. <span class="token number">1</span><span class="token operator">+</span>obj <span class="token comment">//&quot;1[object Object]&quot;</span>
    105. 

  105. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码将对象<code>obj</code>与数字<code>1</code>相加，这时 JavaScript 就会<strong>自动调用</strong><code>valueOf()</code>方法，求出<code>obj</code>的值再与<code>1</code>相加。所以，如果自定义<code>valueOf</code>方法，就可以得到想要的结果。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    106. 

  106. obj<span class="token punctuation">.</span><span class="token function-variable function">valueof</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    107. 

  107.     <span class="token keyword">return</span> <span class="token number">2</span>
    108. 

  108. <span class="token punctuation">}</span>
    109. 

  109. <span class="token number">1</span> <span class="token operator">+</span> obj <span class="token comment">// 3</span>
    110. 

  110. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码自定义了<code>obj</code>对象的<code>valueOf</code>方法，于是<code>1 + obj</code>就得到了<code>3</code>。这种方法就相当于用自定义的<code>obj.valueOf</code>，覆盖<code>Object.prototype.valueOf</code>。</p> <h3 id="object-prototype-tostring"><a href="#object-prototype-tostring" class="header-anchor">#</a> Object.prototype.toString()</h3> <p><code>toString</code>方法的作用是返回一个对象的字符串形式，默认情况下<strong>返回类型字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> o1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    111. 

  111. o1<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Object]&quot;</span>
    112. 

  112. 

  113. <span class="token keyword">var</span> o2 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span><span class="token number">2</span><span class="token punctuation">}</span>
    114. 

  114. o2<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Object]&quot;</span>
    115. 

  115. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码表示，对于一个对象调用<code>toString</code>方法，会返回字符串<code>[object Object]</code>，该字符串说明对象的类型。</p> <p>字符串<code>[object Object]</code>本身没有太大的用处，但是通过自定义<code>toString</code>方法，可以让对象在自动类型转换时，得到想要的字符串形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    116. 

  116. 

  117. obj<span class="token punctuation">.</span><span class="token function-variable function">toString</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    118. 

  118.     <span class="token keyword">return</span> <span class="token string">'hello'</span>
    119. 

  119. <span class="token punctuation">}</span>
    120. 

  120. obj <span class="token operator">+</span> <span class="token string">' '</span><span class="token operator">+</span> <span class="token string">'world'</span> <span class="token comment">// &quot;hello world&quot;</span>
    121. 

  121. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码表示，当对象用于字符串加法时，会<strong>自动调用</strong><code>toString</code>方法。由于自定义了<code>toString</code>方法，所以返回字符串<code>hello world</code>。</p> <p>数组、字符串、函数、Date 对象都分别部署了自定义的<code>toString</code>方法，覆盖了<code>Object.prototype.toString</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1,2,3&quot;</span>
    122. 

  122. 

  123. <span class="token string">'123'</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;123&quot;</span>
    124. 

  124. 

  125. <span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    126. 

  126.   <span class="token keyword">return</span> <span class="token number">123</span><span class="token punctuation">;</span>
    127. 

  127. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    128. 

  128. <span class="token comment">// &quot;function () {</span>
    129. 

  129. <span class="token comment">//   return 123;</span>
    130. 

  130. <span class="token comment">// }&quot;</span>
    131. 

  131. 

  132. <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    133. 

  133. <span class="token comment">// &quot;Tue May 10 2016 09:11:31 GMT+0800 (CST)&quot;</span>
    134. 

  134. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，数组、字符串、函数、Date 对象调用<code>toString</code>方法，并不会返回<code>[object Object]</code>，因为它们都自定义了<code>toString</code>方法，覆盖原始方法。</p> <h4 id="tostring-的应用-判断数据类型"><a href="#tostring-的应用-判断数据类型" class="header-anchor">#</a> toString() 的应用：判断数据类型</h4> <p><code>Object.prototype.toString</code>方法返回对象的类型字符串，因此可以用来判断一个值的类型。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var obj = {};
    135. 

  135. obj.toString() // &quot;[object Object]&quot;
    136. 

  136. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码调用空对象的<code>toString</code>方法，结果返回一个字符串<code>object Object</code>，其中第二个<code>Object</code>表示该值的构造函数。这是一个十分有用的判断数据类型的方法。</p> <p>由于实例对象可能会自定义<code>toString</code>方法，覆盖掉<code>Object.prototype.toString</code>方法，所以为了得到类型字符串，最好直接使用<code>Object.prototype.toString</code>方法。通过函数的<code>call</code>方法，可以在任意值上调用这个方法，帮助我们判断这个值的类型。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>Object.prototype.toString.call(value)
    137. 

  137. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码表示对<code>value</code>这个值调用<code>Object.prototype.toString</code>方法。</p> <p>不同数据类型的<code>Object.prototype.toString</code>方法返回值如下。</p> <ul><li>数值：返回<code>[object Number]</code>。</li> <li>字符串：返回<code>[object String]</code>。</li> <li>布尔值：返回<code>[object Boolean]</code>。</li> <li>undefined：返回<code>[object Undefined]</code>。</li> <li>null：返回<code>[object Null]</code>。</li> <li>数组：返回<code>[object Array]</code>。</li> <li>arguments 对象：返回<code>[object Arguments]</code>。</li> <li>函数：返回<code>[object Function]</code>。</li> <li>Error 对象：返回<code>[object Error]</code>。</li> <li>Date 对象：返回<code>[object Date]</code>。</li> <li>RegExp 对象：返回<code>[object RegExp]</code>。</li> <li>其他对象：返回<code>[object Object]</code>。</li></ul> <p>这就是说，<code>Object.prototype.toString</code>可以看出一个值到底是什么类型。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Number]&quot;</span>
    138. 

  138. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object String]&quot;</span>
    139. 

  139. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Boolean]&quot;</span>
    140. 

  140. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Undefined]&quot;</span>
    141. 

  141. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Null]&quot;</span>
    142. 

  142. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>Math<span class="token punctuation">)</span> <span class="token comment">// &quot;[object Math]&quot;</span>
    143. 

  143. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Object]&quot;</span>
    144. 

  144. <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[object Array]&quot;</span>
    145. 

  145. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>利用这个特性，可以写出一个比<code>typeof</code>运算符更准确的类型判断函数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token function-variable function">type</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">o</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    146. 

  146.     <span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span>
    147. 

  147.     <span class="token keyword">return</span> s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">\[object (.*?)\]</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    148. 

  148. <span class="token punctuation">}</span>
    149. 

  149. <span class="token function">type</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;object&quot;</span>
    150. 

  150. <span class="token function">type</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;array&quot;</span>
    151. 

  151. <span class="token function">type</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;number&quot;</span>
    152. 

  152. <span class="token function">type</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;null&quot;</span>
    153. 

  153. <span class="token function">type</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;undefined&quot;</span>
    154. 

  154. <span class="token function">type</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">abcd</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;regex&quot;</span>
    155. 

  155. <span class="token function">type</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;date&quot;</span>
    156. 

  156. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>在上面这个<code>type</code>函数的基础上，还可以加上专门判断某种类型数据的方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token function-variable function">type</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">o</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    157. 

  157.   <span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">;</span>
    158. 

  158.   <span class="token keyword">return</span> s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">\[object (.*?)\]</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    159. 

  159. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    160. 

  160. 

  161. <span class="token punctuation">[</span><span class="token string">'Null'</span><span class="token punctuation">,</span>
    162. 

  162.  <span class="token string">'Undefined'</span><span class="token punctuation">,</span>
    163. 

  163.  <span class="token string">'Object'</span><span class="token punctuation">,</span>
    164. 

  164.  <span class="token string">'Array'</span><span class="token punctuation">,</span>
    165. 

  165.  <span class="token string">'String'</span><span class="token punctuation">,</span>
    166. 

  166.  <span class="token string">'Number'</span><span class="token punctuation">,</span>
    167. 

  167.  <span class="token string">'Boolean'</span><span class="token punctuation">,</span>
    168. 

  168.  <span class="token string">'Function'</span><span class="token punctuation">,</span>
    169. 

  169.  <span class="token string">'RegExp'</span>
    170. 

  170. <span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">t</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    171. 

  171.   type<span class="token punctuation">[</span><span class="token string">'is'</span> <span class="token operator">+</span> t<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">o</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    172. 

  172.     <span class="token keyword">return</span> <span class="token function">type</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span> <span class="token operator">===</span> t<span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    173. 

  173.   <span class="token punctuation">}</span><span class="token punctuation">;</span>
    174. 

  174. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    175. 

  175. 

  176. type<span class="token punctuation">.</span><span class="token function">isObject</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    177. 

  177. type<span class="token punctuation">.</span><span class="token function">isNumber</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    178. 

  178. type<span class="token punctuation">.</span><span class="token function">isRegExp</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">abc</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    179. 

  179. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><h3 id="object-prototype-tolocalestring"><a href="#object-prototype-tolocalestring" class="header-anchor">#</a> Object.prototype.toLocaleString()</h3> <p><code>Object.prototype.toLocaleString</code>方法与<code>toString</code>的返回结果相同，也是返回一个值的字符串形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    180. 

  180. obj<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// &quot;[object Object]&quot;</span>
    181. 

  181. obj<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// &quot;[object Object]&quot;</span>
    182. 

  182. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这个方法的主要作用是留出一个接口，让各种不同的对象实现自己版本的<code>toLocaleString</code>，用来返回针对某些地域的特定的值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> person <span class="token operator">=</span> <span class="token punctuation">{</span>
    183. 

  183.   <span class="token function-variable function">toString</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    184. 

  184.     <span class="token keyword">return</span> <span class="token string">'Henry Norman Bethune'</span><span class="token punctuation">;</span>
    185. 

  185.   <span class="token punctuation">}</span><span class="token punctuation">,</span>
    186. 

  186.   <span class="token function-variable function">toLocaleString</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    187. 

  187.     <span class="token keyword">return</span> <span class="token string">'白求恩'</span><span class="token punctuation">;</span>
    188. 

  188.   <span class="token punctuation">}</span>
    189. 

  189. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    190. 

  190. 

  191. person<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Henry Norman Bethune</span>
    192. 

  192. person<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 白求恩</span>
    193. 

  193. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>toString()</code>方法返回对象的一般字符串形式，<code>toLocaleString()</code>方法返回本地的字符串形式。</p> <p>目前，主要有三个对象自定义了<code>toLocaleString</code>方法。</p> <ul><li>Array.prototype.toLocaleString()</li> <li>Number.prototype.toLocaleString()</li> <li>Date.prototype.toLocaleString()</li></ul> <p>举例来说，日期的实例对象的<code>toString</code>和<code>toLocaleString</code>返回值就不一样，而且<code>toLocaleString</code>的返回值跟用户设定的所在地域相关。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> date <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    194. 

  194. date<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Sun Oct 06 2019 15:50:04 GMT+0800 (中国标准时间)&quot;</span>
    195. 

  195. date<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;2019/10/6 下午3:50:04&quot;</span>
    196. 

  196. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="object-prototype-hasownproperty"><a href="#object-prototype-hasownproperty" class="header-anchor">#</a> Object.prototype.hasOwnProperty()</h3> <p><code>Object.prototype.hasOwnProperty</code>方法接受一个字符串作为参数，返回一个布尔值，表示该实例对象自身是否具有该属性。</p> <p><strong>检查对象是否包含有某个属性，返回布尔值</strong></p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var obj = {
    197. 

  197.   p: 123
    198. 

  198. };
    199. 

  199. 

  200. obj.hasOwnProperty('p') // true
    201. 

  201. obj.hasOwnProperty('toString') // false
    202. 

  202. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，对象<code>obj</code>自身具有<code>p</code>属性，所以返回<code>true</code>。<code>toString</code>属性是继承的，所以返回<code>false</code>。</p> <h2 id="二、属性描述对象"><a href="#二、属性描述对象" class="header-anchor">#</a> 二、属性描述对象</h2> <h2 id="_1、概述-2"><a href="#_1、概述-2" class="header-anchor">#</a> 1、概述</h2> <p>JavaScript 提供了一个内部数据结构，用来描述对象的属性，控制它的行为，比如该属性是否可写、可遍历等等。这个内部数据结构称为“属性描述对象”（attributes object）。每个属性都有自己对应的属性描述对象，保存该属性的一些元信息。</p> <p>下面是属性描述对象的一个例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">{</span>
    203. 

  203.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span> <span class="token comment">// 该属性的属性值，默认undefined</span>
    204. 

  204.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span> <span class="token comment">// 属性值是否可改变，默认为true</span>
    205. 

  205.   <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token comment">// 该属性是否可遍历，默认为true</span>
    206. 

  206.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span> <span class="token comment">//属性描述对象的可配置性,默认为true</span>
    207. 

  207.   <span class="token literal-property property">get</span><span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token comment">// 该属性的取值函数（getter），默认为undefined</span>
    208. 

  208.   <span class="token literal-property property">set</span><span class="token operator">:</span> <span class="token keyword">undefined</span> <span class="token comment">// 该属性的存值函数（setter），默认为undefined</span>
    209. 

  209. <span class="token punctuation">}</span>
    210. 

  210. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>属性描述对象提供6个元属性。</p> <p>（1）<code>value</code></p> <p><code>value</code>是该属性的属性值，默认为<code>undefined</code>。</p> <p>（2）<code>writable</code></p> <p><code>writable</code>是一个布尔值，表示属性值（value）是否可改变（即是否可写），默认为<code>true</code>。</p> <p>（3）<code>enumerable</code></p> <p><code>enumerable</code>是一个布尔值，表示该属性是否可遍历，默认为<code>true</code>。如果设为<code>false</code>，会使得某些操作（比如<code>for...in</code>循环、<code>Object.keys()</code>）跳过该属性。</p> <p>（4）<code>configurable</code></p> <p><code>configurable</code>是一个布尔值，表示可配置性，默认为<code>true</code>。如果设为<code>false</code>，将阻止某些操作改写该属性，比如无法删除该属性，也不得改变该属性的属性描述对象（<code>value</code>属性除外）。也就是说，<code>configurable</code>属性控制了属性描述对象的可写性。</p> <p>（5）<code>get</code></p> <p><code>get</code>是一个函数，表示该属性的取值函数（getter），默认为<code>undefined</code>。</p> <p>（6）<code>set</code></p> <p><code>set</code>是一个函数，表示该属性的存值函数（setter），默认为<code>undefined</code>。</p> <h2 id="_2、object-getownpropertydescriptor"><a href="#_2、object-getownpropertydescriptor" class="header-anchor">#</a> 2、Object.getOwnPropertyDescriptor()</h2> <p><code>Object.getOwnPropertyDescriptor()</code>方法可以<strong>获取属性描述对象</strong>。它的第一个参数是目标对象，第二个参数是一个字符串，对应目标对象的某个属性名。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span><span class="token string">'a'</span><span class="token punctuation">}</span>
    211. 

  211. 

  212. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span><span class="token string">'p'</span><span class="token punctuation">)</span>
    213. 

  213. <span class="token comment">// Object { value: &quot;a&quot;,  // 属性值'a'</span>
    214. 

  214. <span class="token comment">//   writable: true, // 属性值'a' 可改写</span>
    215. 

  215. <span class="token comment">//   enumerable: true, // 'p'属性可被遍历</span>
    216. 

  216. <span class="token comment">//   configurable: true // 属性描述对象的可配置性</span>
    217. 

  217. <span class="token comment">// }</span>
    218. 

  218. 

  219. 

  220. <span class="token comment">// getOwnPropertyDescriptor 中文含义：获取自己的属性描述符</span>
    221. 

  221. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>注意，<code>Object.getOwnPropertyDescriptor()</code>方法只能用于对象自身的属性，不能用于继承的属性。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'a'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    222. 

  222. 

  223. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'toString'</span><span class="token punctuation">)</span>
    224. 

  224. <span class="token comment">// undefined</span>
    225. 

  225. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>toString</code>是<code>obj</code>对象继承的属性，<code>Object.getOwnPropertyDescriptor()</code>无法获取。</p> <h2 id="_3、object-getownpropertynames"><a href="#_3、object-getownpropertynames" class="header-anchor">#</a> 3、Object.getOwnPropertyNames()</h2> <p><code>Object.getOwnPropertyNames</code>方法返回一个数组，成员是参数对象自身的全部属性的属性名，不管该属性是否可遍历。</p> <p><strong>返回对象的全部属性名</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperties</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    226. 

  226.   <span class="token literal-property property">p1</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    227. 

  227.   <span class="token literal-property property">p2</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span>
    228. 

  228. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    229. 

  229. 

  230. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
    231. 

  231. <span class="token comment">// [&quot;p1&quot;, &quot;p2&quot;]</span>
    232. 

  232. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>obj.p1</code>是可遍历的，<code>obj.p2</code>是不可遍历的。<code>Object.getOwnPropertyNames</code>会将它们都返回。</p> <p>这跟<code>Object.keys</code>的行为不同，<code>Object.keys</code>只返回对象自身的可遍历属性的全部属性名。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
    233. 

  233. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// [ 'length' ]</span>
    234. 

  234. 

  235. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span> <span class="token comment">// []</span>
    236. 

  236. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span>
    237. 

  237.  <span class="token comment">/*[
    238. 

  238. 	&quot;constructor&quot;,   // 构造函数
    239. 

  239.     &quot;__defineGetter__&quot;,
    240. 

  240.     &quot;__defineSetter__&quot;,
    241. 

  241.     &quot;hasOwnProperty&quot;,  // 检测属性名
    242. 

  242.     &quot;__lookupGetter__&quot;,
    243. 

  243.     &quot;__lookupSetter__&quot;,
    244. 

  244.     &quot;isPrototypeOf&quot;,
    245. 

  245.     &quot;propertyIsEnumerable&quot;,
    246. 

  246.     &quot;toString&quot;,  // 返回类型字符串
    247. 

  247.     &quot;valueOf&quot;,
    248. 

  248.     &quot;__proto__&quot;,  // 原型链
    249. 

  249.     &quot;toLocaleString&quot;
    250. 

  250.    ]
    251. 

  251. */</span>
    252. 

  252. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>上面代码中，数组自身的<code>length</code>属性是不可遍历的，<code>Object.keys</code>不会返回该属性。第二个例子的<code>Object.prototype</code>也是一个对象，所有实例对象都会继承它，它自身的属性都是不可遍历的。</p> <h2 id="_4、object-defineproperty-object-defineproperties"><a href="#_4、object-defineproperty-object-defineproperties" class="header-anchor">#</a> 4、Object.defineProperty()，Object.defineProperties()</h2> <p><code>Object.defineProperty()</code>方法允许通过属性描述对象，定义或修改一个属性，然后返回修改后的对象，它的用法如下。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>object<span class="token punctuation">,</span> propertyName<span class="token punctuation">,</span> attributesObject<span class="token punctuation">)</span>
    253. 

  253. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>Object.defineProperty</code>方法接受三个参数，依次如下。</p> <ul><li>object：属性所在的对象</li> <li>propertyName：字符串，表示属性名</li> <li>attributesObject：属性描述对象</li></ul> <p>举例来说，定义<code>obj.p</code>可以写成下面这样。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    254. 

  254.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
    255. 

  255.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span> <span class="token comment">// 设置为false，无法被改写</span>
    256. 

  256.   <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token comment">// 可遍历</span>
    257. 

  257.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span> <span class="token comment">// 属性描述不可再被配置</span>
    258. 

  258. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    259. 

  259. 

  260. obj<span class="token punctuation">.</span>p <span class="token comment">// 123</span>
    261. 

  261. 

  262. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">246</span><span class="token punctuation">;</span> <span class="token comment">// 由于writable为false,改写无效</span>
    263. 

  263. obj<span class="token punctuation">.</span>p <span class="token comment">// 123</span>
    264. 

  264. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>Object.defineProperty()</code>方法定义了<code>obj.p</code>属性。由于属性描述对象的<code>writable</code>属性为<code>false</code>，所以<code>obj.p</code>属性不可写。注意，这里的<code>Object.defineProperty</code>方法的第一个参数是<code>{}</code>（一个新建的空对象），<code>p</code>属性直接定义在这个空对象上面，然后返回这个对象，这是<code>Object.defineProperty()</code>的常见用法。</p> <p>如果属性已经存在，<code>Object.defineProperty()</code>方法相当于更新该属性的属性描述对象。</p> <p>如果<strong>一次性定义或修改多个属性</strong>，可以使用<code>Object.defineProperties()</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperties</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    265. 

  265.   <span class="token literal-property property">p1</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span> <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    266. 

  266.   <span class="token literal-property property">p2</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'abc'</span><span class="token punctuation">,</span> <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    267. 

  267.   <span class="token literal-property property">p3</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>p1 <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>p2 <span class="token punctuation">}</span><span class="token punctuation">,</span>
    268. 

  268.     <span class="token literal-property property">enumerable</span><span class="token operator">:</span><span class="token boolean">true</span><span class="token punctuation">,</span>
    269. 

  269.     <span class="token literal-property property">configurable</span><span class="token operator">:</span><span class="token boolean">true</span>
    270. 

  270.   <span class="token punctuation">}</span>
    271. 

  271. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    272. 

  272. 

  273. obj<span class="token punctuation">.</span>p1 <span class="token comment">// 123</span>
    274. 

  274. obj<span class="token punctuation">.</span>p2 <span class="token comment">// &quot;abc&quot;</span>
    275. 

  275. obj<span class="token punctuation">.</span>p3 <span class="token comment">// &quot;123abc&quot;</span>
    276. 

  276. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，<code>Object.defineProperties()</code>同时定义了<code>obj</code>对象的三个属性。其中，<code>p3</code>属性定义了取值函数<code>get</code>，即每次读取该属性，都会调用这个取值函数。</p> <p><strong>注意</strong>，一旦定义了取值函数<code>get</code>（或存值函数<code>set</code>），就不能将<code>writable</code>属性设为<code>true</code>，或者同时定义<code>value</code>属性，否则会报错。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    277. 

  277. 

  278. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    279. 

  279.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
    280. 

  280.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">456</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    281. 

  281. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    282. 

  282. <span class="token comment">// TypeError: Invalid property.</span>
    283. 

  283. <span class="token comment">// A property cannot both have accessors and be writable or have a value</span>
    284. 

  284. 

  285. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    286. 

  286.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    287. 

  287.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token number">456</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
    288. 

  288. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    289. 

  289. <span class="token comment">// TypeError: Invalid property descriptor.</span>
    290. 

  290. <span class="token comment">// Cannot both specify accessors and a value or writable attribute</span>
    291. 

  291. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，同时定义了<code>get</code>属性和<code>value</code>属性，以及将<code>writable</code>属性设为<code>true</code>，就会报错。</p> <p><code>Object.defineProperty()</code>和<code>Object.defineProperties()</code>参数里面的属性描述对象，<code>writable</code>、<code>configurable</code>、<code>enumerable</code>这三个属性的默认值都为<code>false</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    292. 

  292. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    293. 

  293. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span>
    294. 

  294. <span class="token comment">// {  // 默认值</span>
    295. 

  295. <span class="token comment">//   value: undefined,</span>
    296. 

  296. <span class="token comment">//   writable: false,</span>
    297. 

  297. <span class="token comment">//   enumerable: false,</span>
    298. 

  298. <span class="token comment">//   configurable: false</span>
    299. 

  299. <span class="token comment">// }</span>
    300. 

  300. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，定义<code>obj.foo</code>时用了一个空的属性描述对象，就可以看到各个元属性的默认值。</p> <h2 id="_5、object-prototype-propertyisenumerable"><a href="#_5、object-prototype-propertyisenumerable" class="header-anchor">#</a> 5、Object.prototype.propertyIsEnumerable()</h2> <p>实例对象的<code>propertyIsEnumerable()</code>方法返回一个布尔值，用来<strong>判断某个属性是否可遍历</strong>。注意，这个方法只能用于判断对象自身的属性，对于继承的属性一律返回<code>false</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    301. 

  301. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    302. 

  302. 

  303. obj<span class="token punctuation">.</span><span class="token function">propertyIsEnumerable</span><span class="token punctuation">(</span><span class="token string">'p'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    304. 

  304. obj<span class="token punctuation">.</span><span class="token function">propertyIsEnumerable</span><span class="token punctuation">(</span><span class="token string">'toString'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    305. 

  305. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>obj.p</code>是可遍历的，而<code>obj.toString</code>是继承的属性。</p> <h2 id="_6、元属性"><a href="#_6、元属性" class="header-anchor">#</a> 6、元属性</h2> <p>属性描述对象的各个属性称为“元属性”，因为它们可以看作是控制属性的属性。</p> <h3 id="value目标属性的值"><a href="#value目标属性的值" class="header-anchor">#</a> value目标属性的值</h3> <p><code>value</code>属性是目标属性的值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    306. 

  306. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    307. 

  307. 

  308. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value
    309. 

  309. <span class="token comment">// 123</span>
    310. 

  310. 

  311. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">246</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    312. 

  312. obj<span class="token punctuation">.</span>p <span class="token comment">// 246</span>
    313. 

  313. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码是通过<code>value</code>属性，读取或改写<code>obj.p</code>的例子。</p> <h3 id="writable可写性"><a href="#writable可写性" class="header-anchor">#</a> writable可写性</h3> <p><code>writable</code>属性是一个布尔值，决定了目标属性的值（value）是否可以被改变。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    314. 

  314. 

  315. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    316. 

  316.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">37</span><span class="token punctuation">,</span>
    317. 

  317.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    318. 

  318. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    319. 

  319. 

  320. obj<span class="token punctuation">.</span>a <span class="token comment">// 37</span>
    321. 

  321. obj<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">25</span><span class="token punctuation">;</span> <span class="token comment">// 可写性为false，值无法被修改</span>
    322. 

  322. obj<span class="token punctuation">.</span>a <span class="token comment">// 37</span>
    323. 

  323. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>obj.a</code>的<code>writable</code>属性是<code>false</code>。然后，改变<code>obj.a</code>的值，不会有任何效果。</p> <p><strong>注意</strong>，正常模式下，对<code>writable</code>为<code>false</code>的属性赋值不会报错，只会默默失败。但是，严格模式下会报错，即使对<code>a</code>属性重新赋予一个同样的值。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>'use strict';
    324. 

  324. var obj = {};
    325. 

  325. 

  326. Object.defineProperty(obj, 'a', {
    327. 

  327.   value: 37,
    328. 

  328.   writable: false
    329. 

  329. });
    330. 

  330. 

  331. obj.a = 37;
    332. 

  332. // Uncaught TypeError: Cannot assign to read only property 'a' of object
    333. 

  333. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码是严格模式，对<code>obj.a</code>任何赋值行为都会报错。</p> <p>如果原型对象的某个属性的<code>writable</code>为<code>false</code>，那么子对象将无法自定义这个属性。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> proto <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    334. 

  334.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span>
    335. 

  335.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    336. 

  336. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    337. 

  337. 

  338. <span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>proto<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// obj对象继承proto对象</span>
    339. 

  339. 

  340. obj<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    341. 

  341. obj<span class="token punctuation">.</span>foo <span class="token comment">// 'a'</span>
    342. 

  342. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>proto</code>是原型对象，它的<code>foo</code>属性不可写。<code>obj</code>对象继承<code>proto</code>，也不可以再自定义这个属性了。如果是严格模式，这样做还会抛出一个错误。</p> <p>但是，有一个规避方法，就是通过覆盖属性描述对象，绕过这个限制。原因是这种情况下，原型链会被完全忽视。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> proto <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    343. 

  343.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span>
    344. 

  344.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    345. 

  345. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    346. 

  346. 

  347. <span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    348. 

  348. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    349. 

  349.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'b'</span>
    350. 

  350. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    351. 

  351. 

  352. obj<span class="token punctuation">.</span>foo <span class="token comment">// &quot;b&quot;</span>
    353. 

  353. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h3 id="enumerable可遍历性"><a href="#enumerable可遍历性" class="header-anchor">#</a> enumerable可遍历性</h3> <p><code>enumerable</code>（可遍历性）返回一个布尔值，表示目标属性是否可遍历。</p> <p>JavaScript 的早期版本，<code>for...in</code>循环是基于<code>in</code>运算符的。我们知道，<code>in</code>运算符不管某个属性是对象自身的还是继承的，都会返回<code>true</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    354. 

  354. <span class="token string">'toString'</span> <span class="token keyword">in</span> obj <span class="token comment">// true</span>
    355. 

  355. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>toString</code>不是<code>obj</code>对象自身的属性，但是<code>in</code>运算符也返回<code>true</code>，这导致了<code>toString</code>属性也会被<code>for...in</code>循环遍历。</p> <p>这显然不太合理，后来就引入了“可遍历性”这个概念。只有可遍历的属性，才会被<code>for...in</code>循环遍历，同时还规定<code>toString</code>这一类实例对象继承的原生属性，都是不可遍历的，这样就保证了<code>for...in</code>循环的可用性。</p> <p>具体来说，如果一个属性的<code>enumerable</code>为<code>false</code>，下面三个操作不会取到该属性。</p> <ul><li><code>for..in</code>循环</li> <li><code>Object.keys</code>方法</li> <li><code>JSON.stringify</code>方法</li></ul> <p>因此，<code>enumerable</code>可以用来设置“秘密”属性。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    356. 

  356. 

  357. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'x'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    358. 

  358.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span>
    359. 

  359.   <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    360. 

  360. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    361. 

  361. 

  362. obj<span class="token punctuation">.</span>x <span class="token comment">// 123</span>
    363. 

  363. 

  364. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> key <span class="token keyword">in</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    365. 

  365.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    366. 

  366. <span class="token punctuation">}</span>
    367. 

  367. <span class="token comment">// undefined</span>
    368. 

  368. 

  369. Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>  <span class="token comment">// []</span>
    370. 

  370. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// &quot;{}&quot;</span>
    371. 

  371. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，<code>obj.x</code>属性的<code>enumerable</code>为<code>false</code>，所以一般的遍历操作都无法获取该属性，使得它有点像“秘密”属性，但不是真正的私有属性，还是可以直接获取它的值。</p> <p>注意，<code>for...in</code>循环包括继承的属性，<code>Object.keys</code>方法不包括继承的属性。如果需要获取对象自身的所有属性，不管是否可遍历，可以使用<code>Object.getOwnPropertyNames</code>方法。</p> <p>另外，<code>JSON.stringify</code>方法会排除<code>enumerable</code>为<code>false</code>的属性，有时<strong>可以利用这一点。如果对象的 JSON 格式输出要排除某些属性，就可以把这些属性的<code>enumerable</code>设为<code>false</code>。</strong></p> <h3 id="configurable可配置性"><a href="#configurable可配置性" class="header-anchor">#</a> configurable可配置性</h3> <p><code>configurable</code>(可配置性）返回一个布尔值，决定了<strong>是否可以修改属性描述对象</strong>。也就是说，<code>configurable</code>为<code>false</code>时，<code>value</code>、<code>writable</code>、<code>enumerable</code>和<code>configurable</code>都不能被修改了。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    372. 

  372.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    373. 

  373.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    374. 

  374.   <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    375. 

  375.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    376. 

  376. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    377. 

  377. 

  378. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    379. 

  379. <span class="token comment">// TypeError: Cannot redefine property: p</span>
    380. 

  380. 

  381. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    382. 

  382. <span class="token comment">// TypeError: Cannot redefine property: p</span>
    383. 

  383. 

  384. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    385. 

  385. <span class="token comment">// TypeError: Cannot redefine property: p</span>
    386. 

  386. 

  387. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    388. 

  388. <span class="token comment">// TypeError: Cannot redefine property: p</span>
    389. 

  389. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>上面代码中，<code>obj.p</code>的<code>configurable</code>为<code>false</code>。然后，改动<code>value</code>、<code>writable</code>、<code>enumerable</code>、<code>configurable</code>，结果都报错。</p> <p><strong>注意</strong>，<code>writable</code>只有在<code>false</code>改为<code>true</code>会报错，<code>true</code>改为<code>false</code>是允许的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    390. 

  390.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    391. 

  391.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    392. 

  392. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    393. 

  393. 

  394. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    395. 

  395. <span class="token comment">// 修改成功</span>
    396. 

  396. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>至于<code>value</code>，只要<code>writable</code>和<code>configurable</code>有一个为<code>true</code>，就允许改动。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> o1 <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    397. 

  397.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    398. 

  398.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    399. 

  399.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    400. 

  400. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    401. 

  401. 

  402. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>o1<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    403. 

  403. <span class="token comment">// 修改成功</span>
    404. 

  404. 

  405. <span class="token keyword">var</span> o2 <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    406. 

  406.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    407. 

  407.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    408. 

  408.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span>
    409. 

  409. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    410. 

  410. 

  411. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>o2<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    412. 

  412. <span class="token comment">// 修改成功</span>
    413. 

  413. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>另外，<code>writable</code>为<code>false</code>时，直接给目标属性赋值，不报错，但不会成功。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    414. 

  414.   <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    415. 

  415.   <span class="token literal-property property">writable</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    416. 

  416.   <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    417. 

  417. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    418. 

  418. 

  419. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    420. 

  420. obj<span class="token punctuation">.</span>p <span class="token comment">// 1</span>
    421. 

  421. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>obj.p</code>的<code>writable</code>为<code>false</code>，对<code>obj.p</code>直接赋值不会生效。如果是严格模式，还会报错。</p> <p>可配置性决定了目标属性是否可以被删除（delete）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperties</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    422. 

  422.   <span class="token literal-property property">p1</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    423. 

  423.   <span class="token literal-property property">p2</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">configurable</span><span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span>
    424. 

  424. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    425. 

  425. 

  426. <span class="token keyword">delete</span> obj<span class="token punctuation">.</span>p1 <span class="token comment">// true</span>
    427. 

  427. <span class="token keyword">delete</span> obj<span class="token punctuation">.</span>p2 <span class="token comment">// false</span>
    428. 

  428. 

  429. obj<span class="token punctuation">.</span>p1 <span class="token comment">// undefined</span>
    430. 

  430. obj<span class="token punctuation">.</span>p2 <span class="token comment">// 2</span>
    431. 

  431. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>obj.p1</code>的<code>configurable</code>是<code>true</code>，所以可以被删除，<code>obj.p2</code>就无法删除。</p> <h2 id="_7、存取器-get和set"><a href="#_7、存取器-get和set" class="header-anchor">#</a> 7、存取器（get和set）</h2> <p>除了直接定义以外，属性还可以用存取器（accessor）定义。其中，存值函数称为<code>setter</code>，使用属性描述对象的<code>set</code>属性；取值函数称为<code>getter</code>，使用属性描述对象的<code>get</code>属性。</p> <p><strong>一旦对目标属性定义了存取器，那么存取的时候，都将执行对应的函数</strong>。利用这个功能，可以实现许多高级特性，比如某个属性禁止赋值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    432. 

  432.   <span class="token function-variable function">get</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 获取属性时执行</span>
    433. 

  433.     <span class="token keyword">return</span> <span class="token string">'getter'</span> <span class="token comment">// 这里相当于定义了p属性的值，如果设置了get则不能再设置value</span>
    434. 

  434.   <span class="token punctuation">}</span><span class="token punctuation">,</span>
    435. 

  435.   <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 设置属性时执行</span>
    436. 

  436.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'setter: '</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    437. 

  437.   <span class="token punctuation">}</span>
    438. 

  438. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    439. 

  439. 

  440. obj<span class="token punctuation">.</span>p <span class="token comment">// &quot;getter&quot;</span>
    441. 

  441. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">123</span> <span class="token comment">// &quot;setter: 123&quot;</span>
    442. 

  442. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>obj.p</code>定义了<code>get</code>和<code>set</code>属性。<code>obj.p</code>取值时，就会调用<code>get</code>；赋值时，就会调用<code>set</code>。</p> <p>JavaScript 还提供了存取器的<strong>另一种写法</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    443. 

  443.     <span class="token keyword">get</span> <span class="token function">p</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span> <span class="token comment">// 相当于为obj对象创建一个p属性，并且赋值为‘getter’</span>
    444. 

  444.         <span class="token keyword">return</span> <span class="token string">'getter'</span>
    445. 

  445.     <span class="token punctuation">}</span><span class="token punctuation">,</span>
    446. 

  446.     ser <span class="token function">p</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    447. 

  447.         console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'setter'</span><span class="token operator">+</span>value<span class="token punctuation">)</span>
    448. 

  448.     <span class="token punctuation">}</span>
    449. 

  449. <span class="token punctuation">}</span>
    450. 

  450. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面的写法与定义属性描述对象是等价的，而且使用更广泛。</p> <p>注意，取值函数<code>get</code>不能接受参数，存值函数<code>set</code>只能接受一个参数（即属性的值）。</p> <p>存取器往往用于，属性的值依赖对象内部数据的场合。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span><span class="token punctuation">{</span>
    451. 

  451.   <span class="token literal-property property">$n</span> <span class="token operator">:</span> <span class="token number">5</span><span class="token punctuation">,</span>
    452. 

  452.   <span class="token keyword">get</span> <span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$n<span class="token operator">++</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    453. 

  453.   <span class="token keyword">set</span> <span class="token function">next</span><span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    454. 

  454.     <span class="token keyword">if</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$n<span class="token punctuation">)</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$n <span class="token operator">=</span> n<span class="token punctuation">;</span>
    455. 

  455.     <span class="token keyword">else</span> <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'新的值必须大于当前值'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    456. 

  456.   <span class="token punctuation">}</span>
    457. 

  457. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    458. 

  458. 

  459. obj<span class="token punctuation">.</span>next <span class="token comment">// 5</span>
    460. 

  460. 

  461. obj<span class="token punctuation">.</span>next <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    462. 

  462. obj<span class="token punctuation">.</span>next <span class="token comment">// 10</span>
    463. 

  463. 

  464. obj<span class="token punctuation">.</span>next <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">;</span>
    465. 

  465. <span class="token comment">// Uncaught Error: 新的值必须大于当前值</span>
    466. 

  466. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，<code>next</code>属性的存值函数和取值函数，都依赖于内部属性<code>$n</code>。</p> <blockquote><p>vue框架就是通过存取器来实现的数据响应式</p></blockquote> <h2 id="_8、对象的拷贝"><a href="#_8、对象的拷贝" class="header-anchor">#</a> 8、对象的拷贝</h2> <p>有时，我们需要将一个对象的所有属性，拷贝到另一个对象，可以用下面的方法实现。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token function-variable function">extend</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">to<span class="token punctuation">,</span> <span class="token keyword">from</span></span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    467. 

  467.     <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> property <span class="token keyword">in</span> from<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    468. 

  468.         to<span class="token punctuation">[</span>property<span class="token punctuation">]</span> <span class="token operator">=</span> from<span class="token punctuation">[</span>property<span class="token punctuation">]</span>
    469. 

  469.     <span class="token punctuation">}</span>
    470. 

  470.     <span class="token keyword">return</span> to
    471. 

  471. <span class="token punctuation">}</span>
    472. 

  472. <span class="token function">extend</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// {a:1}</span>
    473. 

  473. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面这个方法的问题在于，如果遇到存取器定义的属性，会只拷贝到属性和值，并不能拷贝到存取器</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>extend({}, {
    474. 

  474.   get a() { return 1 }
    475. 

  475. })
    476. 

  476. // {a: 1}   // 这里并没有拷贝到get方法
    477. 

  477. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>为了解决这个问题，我们可以通过<code>Object.defineProperty</code>方法来拷贝属性。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token function-variable function">extend</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">to<span class="token punctuation">,</span> <span class="token keyword">from</span></span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    478. 

  478.     <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> property <span class="token keyword">in</span> from<span class="token punctuation">)</span><span class="token punctuation">{</span>
    479. 

  479.     	<span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>from<span class="token punctuation">.</span><span class="token function">hasOwnProperty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token keyword">continue</span> <span class="token comment">//过滤继承属性 continue 终止本次循环，继续剩余循环</span>
    480. 

  480.         Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>
    481. 

  481.             to<span class="token punctuation">,</span>
    482. 

  482.             property<span class="token punctuation">,</span>
    483. 

  483.             Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>from<span class="token punctuation">,</span>property<span class="token punctuation">)</span>
    484. 

  484.         <span class="token punctuation">)</span>
    485. 

  485.     <span class="token punctuation">}</span>
    486. 

  486.     <span class="token keyword">return</span> to
    487. 

  487. <span class="token punctuation">}</span>
    488. 

  488. <span class="token function">extend</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span><span class="token punctuation">{</span><span class="token keyword">get</span> <span class="token function">a</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// {get a(){return 1}}</span>
    489. 

  489. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，<code>hasOwnProperty</code>那一行用来过滤掉继承的属性，否则可能会报错，因为<code>Object.getOwnPropertyDescriptor</code>读不到继承属性的属性描述对象。</p> <h2 id="_9、控制对象读写状态"><a href="#_9、控制对象读写状态" class="header-anchor">#</a> 9、控制对象读写状态</h2> <p>有时需要<strong>冻结对象的读写状态，防止对象被改变</strong>。</p> <p>JavaScript 提供了三种冻结方法：</p> <ul><li><p>最弱的一种是<code>Object.preventExtensions</code> (不能新增，但能被删除和修改)</p></li> <li><p>其次是<code>Object.seal</code> (不能新增和删除，但能修改)</p></li> <li><p>最强的是<code>Object.freeze</code> (不能新增、删除和修改)</p></li></ul> <h3 id="object-preventextensions-防止新增"><a href="#object-preventextensions-防止新增" class="header-anchor">#</a> Object.preventExtensions() 防止新增</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    490. 

  490. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// prevent Extensions 中文： 防止扩展</span>
    491. 

  491. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span><span class="token string">'p'</span><span class="token punctuation">,</span><span class="token punctuation">{</span>
    492. 

  492.     <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'hello'</span>
    493. 

  493. <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// TypeError:cannot define property:p, object is not extensible</span>
    494. 

  494. 

  495. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token number">1</span>
    496. 

  496. obj<span class="token punctuation">.</span>p <span class="token comment">// undefined</span>
    497. 

  497. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>obj</code>对象经过<code>Object.preventExtensions</code>以后，就无法添加新属性了。</p> <h3 id="object-isextensible-是可新增的"><a href="#object-isextensible-是可新增的" class="header-anchor">#</a> Object.isExtensible() 是可新增的？</h3> <p><code>Object.isExtensible</code>方法用于<strong>检查一个对象是否使用了<code>Object.preventExtensions</code>方法</strong>。也就是说，检查是否可以为一个对象添加属性。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    498. 

  498. Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// true    is Extensible 中文： 是可扩展的？</span>
    499. 

  499. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
    500. 

  500. Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// false</span>
    501. 

  501. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，对<code>obj</code>对象使用<code>Object.preventExtensions</code>方法以后，再使用<code>Object.isExtensible</code>方法，返回<code>false</code>，表示已经不能添加新属性了。</p> <h3 id="object-seal-密封"><a href="#object-seal-密封" class="header-anchor">#</a> Object.seal() 密封</h3> <p><code>Object.seal</code>方法使得一个对象<strong>既无法添加新属性，也无法删除旧属性。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'hello'</span><span class="token punctuation">}</span>
    502. 

  502. Object<span class="token punctuation">.</span><span class="token function">seal</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// seal 中文： 密封</span>
    503. 

  503. 

  504. <span class="token keyword">delete</span> obj<span class="token punctuation">.</span>p <span class="token comment">// 删除属性无效</span>
    505. 

  505. obj<span class="token punctuation">.</span>p <span class="token comment">// hello</span>
    506. 

  506. 

  507. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token string">'hi'</span> <span class="token comment">// 可修改值</span>
    508. 

  508. obj<span class="token punctuation">.</span>p <span class="token comment">// hi</span>
    509. 

  509. 

  510. obj<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token string">'world'</span> <span class="token comment">// 新增属性无效</span>
    511. 

  511. obj<span class="token punctuation">.</span>x <span class="token comment">// undefined</span>
    512. 

  512. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>obj</code>对象执行<code>Object.seal</code>方法以后，就无法添加新属性和删除旧属性了。</p> <p><code>Object.seal</code>实质是把属性描述对象的<code>configurable</code>属性设为<code>false</code>，因此属性描述对象不再能改变了。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">}</span>
    513. 

  513. 

  514. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">)</span>
    515. 

  515. <span class="token comment">// Object {</span>
    516. 

  516. <span class="token comment">//   value: &quot;a&quot;,</span>
    517. 

  517. <span class="token comment">//   writable: true,</span>
    518. 

  518. <span class="token comment">//   enumerable: true,</span>
    519. 

  519. <span class="token comment">//   configurable: true</span>
    520. 

  520. <span class="token comment">// }</span>
    521. 

  521. 

  522. Object<span class="token punctuation">.</span><span class="token function">seal</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
    523. 

  523. 

  524. Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'p'</span><span class="token punctuation">)</span>
    525. 

  525. <span class="token comment">// Object {</span>
    526. 

  526. <span class="token comment">//   value: &quot;a&quot;,</span>
    527. 

  527. <span class="token comment">//   writable: true,</span>
    528. 

  528. <span class="token comment">//   enumerable: true,</span>
    529. 

  529. <span class="token comment">//   configurable: false</span>
    530. 

  530. <span class="token comment">// }</span>
    531. 

  531. 

  532. Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span><span class="token string">'p'</span><span class="token punctuation">,</span><span class="token punctuation">{</span>
    533. 

  533.     <span class="token literal-property property">enumerable</span><span class="token operator">:</span>
    534. 

  534. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    535. 

  535. <span class="token comment">// TypeError: Cannot redefine property: p</span>
    536. 

  536. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br></div></div><p>上面代码中，使用<code>Object.seal</code>方法之后，属性描述对象的<code>configurable</code>属性就变成了<code>false</code>，然后改变<code>enumerable</code>属性就会报错。</p> <p><code>Object.seal</code>只是禁止新增或删除属性，并不影响修改某个属性的值。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var obj = { p: 'a' };
    537. 

  537. Object.seal(obj);
    538. 

  538. obj.p = 'b';
    539. 

  539. obj.p // 'b'
    540. 

  540. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>Object.seal</code>方法对<code>p</code>属性的<code>value</code>无效，是因为此时<code>p</code>属性的可写性由<code>writable</code>决定。</p> <h3 id="object-issealed-是密封的"><a href="#object-issealed-是密封的" class="header-anchor">#</a> Object.isSealed() 是密封的？</h3> <p><code>Object.isSealed</code>方法用于<strong>检查一个对象是否使用了<code>Object.seal</code>方法</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span><span class="token string">'a'</span><span class="token punctuation">}</span>
    541. 

  541. Object<span class="token punctuation">.</span><span class="token function">seal</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
    542. 

  542. Object<span class="token punctuation">.</span><span class="token function">isSealed</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// true   is Sealed 中文： 是密封的？</span>
    543. 

  543. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这时，<code>Object.isExtensible</code>方法也返回<code>false</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'a'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    544. 

  544. 

  545. Object<span class="token punctuation">.</span><span class="token function">seal</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    546. 

  546. Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// false</span>
    547. 

  547. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><blockquote><p>isSealed为true，是密封的。isExtensible为false，不是可扩展的</p></blockquote> <h3 id="object-freeze-冻结"><a href="#object-freeze-冻结" class="header-anchor">#</a> Object.freeze() 冻结</h3> <p><code>Object.freeze</code>方法可以使得一个对象<strong>无法添加新属性、无法删除旧属性、也无法改变属性的值</strong>，使得这个对象实际上变成了常量。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">p</span><span class="token operator">:</span><span class="token string">'hello'</span><span class="token punctuation">}</span>
    548. 

  548. Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span>
    549. 

  549. 

  550. obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token string">'world'</span>
    551. 

  551. obj<span class="token punctuation">.</span>p <span class="token comment">// hello</span>
    552. 

  552. 

  553. obj<span class="token punctuation">.</span>t <span class="token operator">=</span> <span class="token string">'hello'</span>
    554. 

  554. obj<span class="token punctuation">.</span>t <span class="token comment">// nudefined</span>
    555. 

  555. 

  556. <span class="token keyword">delete</span> obj<span class="token punctuation">.</span>p <span class="token comment">// false</span>
    557. 

  557. obj<span class="token punctuation">.</span>p <span class="token comment">// hello</span>
    558. 

  558. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，对<code>obj</code>对象进行<code>Object.freeze()</code>以后，修改属性、新增属性、删除属性都无效了。这些操作并不报错，只是默默地失败。如果在严格模式下，则会报错。</p> <h3 id="object-isfrozen-是冻结的"><a href="#object-isfrozen-是冻结的" class="header-anchor">#</a> Object.isFrozen() 是冻结的？</h3> <p><code>Object.isFrozen</code>方法用于<strong>检查一个对象是否使用了<code>Object.freeze</code>方法。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    559. 

  559.   <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'hello'</span>
    560. 

  560. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    561. 

  561. 

  562. Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    563. 

  563. Object<span class="token punctuation">.</span><span class="token function">isFrozen</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// true // 是冻结的</span>
    564. 

  564. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>使用<code>Object.freeze</code>方法以后，<code>Object.isSealed</code>将会返回<code>true</code>，<code>Object.isExtensible</code>返回<code>false</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    565. 

  565.   <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'hello'</span>
    566. 

  566. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    567. 

  567. 

  568. Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    569. 

  569. 

  570. Object<span class="token punctuation">.</span><span class="token function">isSealed</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// true // 是密封的</span>
    571. 

  571. Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// false // 不是可扩展的</span>
    572. 

  572. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p><code>Object.isFrozen</code>的一个<strong>用途</strong>是，确认某个对象没有被冻结后，再对它的属性赋值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    573. 

  573.   <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token string">'hello'</span>
    574. 

  574. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    575. 

  575. 

  576. Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    577. 

  577. 

  578. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>Object<span class="token punctuation">.</span><span class="token function">isFrozen</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    579. 

  579.   obj<span class="token punctuation">.</span>p <span class="token operator">=</span> <span class="token string">'world'</span><span class="token punctuation">;</span>
    580. 

  580. <span class="token punctuation">}</span>
    581. 

  581. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，确认<code>obj</code>没有被冻结后，再对它的属性赋值，就不会报错了。</p> <h3 id="局限性"><a href="#局限性" class="header-anchor">#</a> 局限性</h3> <p>上面的三个方法锁定对象的可写性有一个<strong>漏洞</strong>：<strong>可以通过改变原型对象，来为对象增加属性</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    582. 

  582. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    583. 

  583. 

  584. <span class="token keyword">var</span> proto <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 获取原型对象</span>
    585. 

  585. proto<span class="token punctuation">.</span>t <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    586. 

  586. obj<span class="token punctuation">.</span>t
    587. 

  587. <span class="token comment">// hello</span>
    588. 

  588. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，对象<code>obj</code>本身不能新增属性，但是可以在它的原型对象上新增属性，就依然能够在<code>obj</code>上读到。</p> <p>一种解决方案是，把<code>obj</code>的原型也冻结住。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    589. 

  589. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    590. 

  590. 

  591. <span class="token keyword">var</span> proto <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    592. 

  592. Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>proto<span class="token punctuation">)</span><span class="token punctuation">;</span>
    593. 

  593. 

  594. proto<span class="token punctuation">.</span>t <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    595. 

  595. obj<span class="token punctuation">.</span>t <span class="token comment">// undefined</span>
    596. 

  596. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p><strong>另外一个局限</strong>是，如果属性值是对象，上面这些方法只能冻结属性指向的对象，而不能冻结对象本身的内容。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    597. 

  597.   <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    598. 

  598.   <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span>
    599. 

  599. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    600. 

  600. Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    601. 

  601. 

  602. obj<span class="token punctuation">.</span>bar<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token string">'c'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    603. 

  603. obj<span class="token punctuation">.</span>bar <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;]</span>
    604. 

  604. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>obj.bar</code>属性指向一个数组，<code>obj</code>对象被冻结以后，这个指向无法改变，即无法指向其他值，但是所指向的数组是可以改变的。</p> <h2 id="三、array对象"><a href="#三、array对象" class="header-anchor">#</a> 三、Array对象</h2> <h2 id="_1、array构造函数"><a href="#_1、array构造函数" class="header-anchor">#</a> 1、Array构造函数</h2> <p><code>Array</code>是 JavaScript 的原生对象，同时也是一个构造函数，可以用它生成新的数组。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Array是原生对象，也是构造函数； 2表示生成2个成员</span>
    605. 

  605. arr<span class="token punctuation">.</span>length <span class="token comment">// 2</span>
    606. 

  606. arr <span class="token comment">// [ empty x 2 ]</span>
    607. 

  607. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>Array</code>构造函数的参数<code>2</code>，表示生成一个两个成员的数组，每个位置都是空值。</p> <p>如果没有使用<code>new</code>，运行结果也是一样的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    608. 

  608. <span class="token comment">// 等同于</span>
    609. 

  609. <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token function">Array</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    610. 

  610. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>Array</code>构造函数有一个很大的<strong>缺陷</strong>，就是不同的参数，会导致它的行为不一致。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 无参数时，返回一个空数组</span>
    611. 

  611. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
    612. 

  612. 

  613. <span class="token comment">// 单个正整数参数，表示返回的新数组的长度</span>
    614. 

  614. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// [ empty ]</span>
    615. 

  615. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [ empty x 2 ]</span>
    616. 

  616. 

  617. <span class="token comment">// 非正整数的数值作为参数，会报错</span>
    618. 

  618. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// RangeError: Invalid array length</span>
    619. 

  619. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// RangeError: Invalid array length</span>
    620. 

  620. 

  621. <span class="token comment">// 单个非数值（比如字符串、布尔值、对象等）作为参数，</span>
    622. 

  622. <span class="token comment">// 则该参数是返回的新数组的成员</span>
    623. 

  623. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span> <span class="token comment">// ['abc']</span>
    624. 

  624. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// [Array[1]]</span>
    625. 

  625. 

  626. <span class="token comment">// 多参数时，所有参数都是返回的新数组的成员</span>
    627. 

  627. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [1, 2]</span>
    628. 

  628. <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">)</span> <span class="token comment">// ['a', 'b', 'c']</span>
    629. 

  629. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>可以看到，<code>Array</code>作为构造函数，行为很不一致。因此，不建议使用它生成新数组，直接使用数组字面量是更好的做法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// bad</span>
    630. 

  630. <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    631. 

  631. 

  632. <span class="token comment">// good</span>
    633. 

  633. <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    634. 

  634. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>注意，如果参数是一个正整数，返回数组的成员都是空位。虽然读取的时候返回<code>undefined</code>，但实际上该位置没有任何值。虽然可以取到<code>length</code>属性，但是取不到键名。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Array</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    635. 

  635. <span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    636. 

  636. 

  637. a<span class="token punctuation">.</span>length <span class="token comment">// 3</span>
    638. 

  638. b<span class="token punctuation">.</span>length <span class="token comment">// 3</span>
    639. 

  639. 

  640. a<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token comment">// undefined</span>
    641. 

  641. b<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token comment">// undefined</span>
    642. 

  642. 

  643. <span class="token number">0</span> <span class="token keyword">in</span> a <span class="token comment">// false  // in运算符用于数组时，前面的参数为数组索引</span>
    644. 

  644. <span class="token number">0</span> <span class="token keyword">in</span> b <span class="token comment">// true</span>
    645. 

  645. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>a</code>是一个长度为3的空数组，<code>b</code>是一个三个成员都是<code>undefined</code>的数组。读取键值的时候，<code>a</code>和<code>b</code>都返回<code>undefined</code>，但是<code>a</code>的键位都是空的，<code>b</code>的键位是有值的。</p> <h2 id="_2、静态方法"><a href="#_2、静态方法" class="header-anchor">#</a> 2、静态方法</h2> <h3 id="array-isarray"><a href="#array-isarray" class="header-anchor">#</a> Array.isArray()</h3> <p><code>Array.isArray</code>方法返回一个布尔值，表示参数是否为数组。它可以弥补<code>typeof</code>运算符的不足。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span>
    646. 

  646. <span class="token keyword">typeof</span> arr <span class="token comment">// object // 使用typeof返回的是object</span>
    647. 

  647. Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span><span class="token comment">// true</span>
    648. 

  648. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>typeof</code>运算符只能显示数组的类型是<code>Object</code>，而<code>Array.isArray</code>方法可以识别数组。</p> <h2 id="_3、实例方法"><a href="#_3、实例方法" class="header-anchor">#</a> 3、实例方法</h2> <h3 id="valueof-原数组-tostring-转字符串"><a href="#valueof-原数组-tostring-转字符串" class="header-anchor">#</a> valueOf() 原数组，toString() 转字符串</h3> <p><code>valueOf</code>方法是一个所有对象都拥有的方法，表示对该对象求值。不同对象的<code>valueOf</code>方法不尽一致，数组的<code>valueOf</code>方法<strong>返回数组本身</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span>
    649. 

  649. arr<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// [1,2,3]</span>
    650. 

  650. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>toString</code>方法也是对象的通用方法，数组的<code>toString</code>方法返回数组的字符串形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    651. 

  651. arr<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1,2,3&quot;</span>
    652. 

  652. 

  653. <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    654. 

  654. arr<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1,2,3,4,5,6&quot;</span>
    655. 

  655. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="push-末尾添加-pop-删除末尾"><a href="#push-末尾添加-pop-删除末尾" class="header-anchor">#</a> push() 末尾添加，pop() 删除末尾</h3> <p><code>push</code>方法用于在数组的<strong>末端添加一个或多个元素</strong>，并返回添加新元素后的数组长度。注意，该方法会改变原数组。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr<span class="token operator">=</span><span class="token punctuation">[</span><span class="token punctuation">]</span>
    656. 

  656. arr<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    657. 

  657. arr<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    658. 

  658. arr<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">,</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    659. 

  659. arr <span class="token comment">// [1,'a',true,{}]</span>
    660. 

  660. 

  661. <span class="token comment">// 注：push()方法 返回添加后的数组长度，会改变原数组</span>
    662. 

  662. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码使用<code>push</code>方法，往数组中添加了四个成员。</p> <p><code>pop</code>方法用于<strong>删除数组的最后一个元素</strong>，并返回该元素。注意，该方法会改变原数组。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">,</span><span class="token string">'c'</span><span class="token punctuation">]</span>
    663. 

  663. arr<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 'c'  删除最后一个元素，并返回该元素</span>
    664. 

  664. arr <span class="token comment">// ['a','b']</span>
    665. 

  665. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>对空数组使用<code>pop</code>方法，不会报错，而是返回<code>undefined</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// undefined</span>
    666. 

  666. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>push</code>和<code>pop</code>结合使用，就构成了“后进先出”的<strong>栈结构</strong>（stack）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    667. 

  667. arr<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    668. 

  668. arr<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    669. 

  669. arr<span class="token punctuation">.</span><span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    670. 

  670. arr <span class="token comment">// [1, 2]</span>
    671. 

  671. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>3</code>是最后进入数组的，但是最早离开数组。</p> <h3 id="unshift-首位添加-shift-删除首位"><a href="#unshift-首位添加-shift-删除首位" class="header-anchor">#</a> unshift() 首位添加, shift() 删除首位</h3> <p><code>unshift()</code>方法用于在数组的<strong>第一个位置添加元素</strong>，并返回添加新元素后的数组长度。注意，该方法会改变原数组。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    672. 

  672. 

  673. a<span class="token punctuation">.</span><span class="token function">unshift</span><span class="token punctuation">(</span><span class="token string">'x'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 4</span>
    674. 

  674. a <span class="token comment">// ['x', 'a', 'b', 'c']</span>
    675. 

  675. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>unshift()</code>方法可以接受多个参数，这些参数都会添加到目标数组头部。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'d'</span> <span class="token punctuation">]</span><span class="token punctuation">;</span>
    676. 

  676. arr<span class="token punctuation">.</span><span class="token function">unshift</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    677. 

  677. arr <span class="token comment">// [ 'a', 'b', 'c', 'd' ]</span>
    678. 

  678. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>shift()</code>方法用于<strong>删除数组的第一个元素</strong>，并返回该元素。注意，该方法会改变原数组。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">,</span><span class="token string">'c'</span><span class="token punctuation">]</span>
    679. 

  679. a<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 'a' 删除第一个元素，并返回该元素</span>
    680. 

  680. a <span class="token comment">// ['b','c']</span>
    681. 

  681. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，使用<code>shift()</code>方法以后，原数组就变了。</p> <p><code>shift()</code>方法可以遍历并清空一个数组。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> list <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    682. 

  682. <span class="token keyword">var</span> item<span class="token punctuation">;</span>
    683. 

  683. 

  684. <span class="token keyword">while</span> <span class="token punctuation">(</span>item <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    685. 

  685.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
    686. 

  686. <span class="token punctuation">}</span>
    687. 

  687. 

  688. list <span class="token comment">// []</span>
    689. 

  689. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码通过<code>list.shift()</code>方法每次取出一个元素，从而遍历数组。它的前提是数组元素不能是<code>0</code>或任何布尔值等于<code>false</code>的元素，因此<strong>这样的遍历不是很可靠</strong>。</p> <p><code>push()</code>和<code>shift()</code>结合使用，就构成了“先进先出”的<strong>队列(堆)结构</strong>（queue）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> queue <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
    690. 

  690. queue<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token string">'1'</span><span class="token punctuation">)</span>
    691. 

  691. queue<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token string">'2'</span><span class="token punctuation">)</span>
    692. 

  692. queue<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token string">'3'</span><span class="token punctuation">)</span>
    693. 

  693. queue<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    694. 

  694. queue <span class="token comment">// ['2','3']</span>
    695. 

  695. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><blockquote><p>push() 最后位置添加元素，unshift() 第一位置添加元素</p> <p>pop() 删除最后一个元素，shift() 删除第一个元素</p></blockquote> <h3 id="join-连接每个成员-转字符串"><a href="#join-连接每个成员-转字符串" class="header-anchor">#</a> join() 连接每个成员-转字符串</h3> <p><code>join()</code>方法以指定参数作为分隔符，<strong>将所有数组成员连接为一个字符串返回</strong>。如果不提供参数，默认用逗号分隔。</p> <p><strong>数组转字符串</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">]</span>
    696. 

  696. 

  697. a<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span> <span class="token comment">// '1234'</span>
    698. 

  698. a<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">' '</span><span class="token punctuation">)</span> <span class="token comment">// '1 2 3 4'</span>
    699. 

  699. a<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">)</span> <span class="token comment">// '1|2|3|4‘</span>
    700. 

  700. a<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// '1,2,3,4'</span>
    701. 

  701. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>如果数组成员是<code>undefined</code>或<code>null</code>或空位，会被转成空字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">'#'</span><span class="token punctuation">)</span>
    702. 

  702. <span class="token comment">// '#'</span>
    703. 

  703. 

  704. <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">'-'</span><span class="token punctuation">)</span>
    705. 

  705. <span class="token comment">// 'a--b'</span>
    706. 

  706. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>通过<code>call</code>方法，这个方法也可以用于字符串或类似数组的对象。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">,</span> <span class="token string">'-'</span><span class="token punctuation">)</span>
    707. 

  707. <span class="token comment">// &quot;h-e-l-l-o&quot;</span>
    708. 

  708. <span class="token comment">// 等价 'hello'.split('').join('-')</span>
    709. 

  709. 

  710. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token number">0</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// 数组的变形对象</span>
    711. 

  711. <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token string">'-'</span><span class="token punctuation">)</span>
    712. 

  712. <span class="token comment">// 'a-b'</span>
    713. 

  713. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="concat-合并数组"><a href="#concat-合并数组" class="header-anchor">#</a> concat() 合并数组</h3> <p><code>concat</code>方法用于<strong>多个数组的合并</strong>。它将新数组的成员，添加到原数组成员的后部，然后<strong>返回一个新数组，原数组不变</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token string">'hello'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'world'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    714. 

  714. <span class="token comment">// ['hello','world']</span>
    715. 

  715. 

  716. <span class="token punctuation">[</span><span class="token string">'hello'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'world'</span><span class="token punctuation">]</span><span class="token punctuation">,</span><span class="token punctuation">[</span><span class="token string">'!'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    717. 

  717. <span class="token comment">// [&quot;hello&quot;, &quot;world&quot;, &quot;!&quot;]</span>
    718. 

  718. 

  719. <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    720. 

  720. <span class="token comment">// [{ a: 1 }, { b: 2 }]</span>
    721. 

  721. 

  722. <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
    723. 

  723. <span class="token comment">// [2, {a: 1}]</span>
    724. 

  724. 

  725. <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">)</span>
    726. 

  726. <span class="token comment">// [1,2]</span>
    727. 

  727. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>除了数组作为参数，<code>concat</code>也接受其他类型的值作为参数，添加到目标数组尾部。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span>
    728. 

  728. <span class="token comment">// [1, 2, 3, 4, 5, 6]</span>
    729. 

  729. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果数组成员包括对象，<code>concat</code>方法返回当前数组的一个浅拷贝。所谓“<strong>浅拷贝</strong>”，指的是新数组拷贝的是对象的引用。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    730. 

  730. <span class="token keyword">var</span> oldArray <span class="token operator">=</span> <span class="token punctuation">[</span>obj<span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// 浅拷贝</span>
    731. 

  731. 

  732. <span class="token keyword">var</span> newArray <span class="token operator">=</span> oldArray<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    733. 

  733. 

  734. obj<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// 改变原对象以后，新数组跟着改变</span>
    735. 

  735. newArray<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span>a <span class="token comment">// 2</span>
    736. 

  736. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，原数组包含一个对象，<code>concat</code>方法生成的新数组包含这个对象的引用。所以，<strong>改变原对象以后，新数组跟着改变</strong>。</p> <h3 id="reverse-反转"><a href="#reverse-反转" class="header-anchor">#</a> reverse() 反转</h3> <p><code>reverse</code>方法用于<strong>颠倒排列数组元素</strong>，返回改变后的数组。注意，该方法将<strong>改变原数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">]</span>
    737. 

  737. a<span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// [4,3,2,1]</span>
    738. 

  738. a <span class="token comment">// [4,3,2,1]</span>
    739. 

  739. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="slice-部分-提取部分成员"><a href="#slice-部分-提取部分成员" class="header-anchor">#</a> slice() 部分-提取部分成员</h3> <p><code>slice</code>方法用于<strong>提取目标数组的一部分</strong>，返回一个新数组，<strong>原数组不变</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>arr<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span>start<span class="token punctuation">,</span> end<span class="token punctuation">)</span><span class="token punctuation">;</span>
    740. 

  740. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>它的第一个参数为起始位置（从0开始），第二个参数为终止位置（但该位置的元素本身不包括在内）。如果省略第二个参数，则一直返回到原数组的最后一个成员。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">,</span><span class="token string">'c'</span><span class="token punctuation">]</span>
    741. 

  741. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// ['a','b','c'] 从0到最后一个成员</span>
    742. 

  742. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// ['b','c'] 从1到最后一个成员</span>
    743. 

  743. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// ['b'] 从1到2成员，不包含2成员</span>
    744. 

  744. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// ['c'] 从2到6成员，超出的不计算在内</span>
    745. 

  745. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">//['a','b','c'] 无参数，默认全部，多用于拷贝数组到新的数组</span>
    746. 

  746. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，最后一个例子<code>slice</code>没有参数，实际上等于返回一个原数组的拷贝。</p> <p>如果<code>slice</code>方法的参数是<strong>负数</strong>，则表示<strong>倒数计算的位置</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    747. 

  747. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;b&quot;, &quot;c&quot;]</span>
    748. 

  748. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;b&quot;]</span>
    749. 

  749. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>-2</code>表示倒数计算的第二个位置，<code>-1</code>表示倒数计算的第一个位置。</p> <p>如果<strong>第一个参数大于等于数组长度</strong>，或者<strong>第二个参数小于第一个参数</strong>，则<strong>返回空数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    750. 

  750. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
    751. 

  751. a<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
    752. 

  752. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h4 id="应用-类数组转数组"><a href="#应用-类数组转数组" class="header-anchor">#</a> 应用-类数组转数组</h4> <p>slice`方法的<strong>一个重要应用</strong>，是<strong>将类似数组的对象转为真正的数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token number">0</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token literal-property property">length</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    753. 

  753. <span class="token comment">// ['a', 'b']</span>
    754. 

  754. 

  755. <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">&quot;div&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    756. 

  756. <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
    757. 

  757. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码的参数都不是数组，但是通过<code>call</code>方法，在它们上面调用<code>slice</code>方法，就可以把它们转为真正的数组。</p> <h3 id="splice-从第几个开始-删除几个-并添加成员-返回被删除成员。"><a href="#splice-从第几个开始-删除几个-并添加成员-返回被删除成员。" class="header-anchor">#</a> splice()  从第几个开始，删除几个，并添加成员，返回被删除成员。</h3> <p><code>splice</code>方法用于<strong>删除原数组的一部分成员</strong>，<strong>并可以在删除的位置添加新的数组成员</strong>，<strong>返回值是被删除的元素</strong>。注意，该方法会<strong>改变原数组</strong>。</p> <blockquote><p>也可以不删除成员同时在指定位置添加新成员</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code>arr<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span>start<span class="token punctuation">,</span> count<span class="token punctuation">,</span> addElement1<span class="token punctuation">,</span> addElement2<span class="token punctuation">,</span> <span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    758. 

  758. <span class="token comment">// 第几个开始，删除几个，加入的新成员...(非必需）</span>
    759. 

  759. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>splice</code>的第一个参数是删除的起始位置（从0开始），第二个参数是被删除的元素个数。如果后面还有更多的参数，则表示这些就是要被插入数组的新元素。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">6</span><span class="token punctuation">]</span>
    760. 

  760. a<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [3,4] 返回被删除成员</span>
    761. 

  761. a<span class="token punctuation">.</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">6</span><span class="token punctuation">]</span>
    762. 

  762. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码从原数组2号位置，删除了2个数组成员。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'d'</span><span class="token punctuation">,</span> <span class="token string">'e'</span><span class="token punctuation">,</span> <span class="token string">'f'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    763. 

  763. a<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;e&quot;, &quot;f&quot;]</span>
    764. 

  764. a <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;, &quot;d&quot;, 1, 2]</span>
    765. 

  765. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码从原数组4号位置删除2个成员，并插入了[1 , 2]新成员。</p> <p>起始位置如果是负数，就表示从倒数位置开始删除。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'d'</span><span class="token punctuation">,</span> <span class="token string">'e'</span><span class="token punctuation">,</span> <span class="token string">'f'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    766. 

  766. a<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;c&quot;, &quot;d&quot;] 负数表示倒数</span>
    767. 

  767. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示，从倒数第四个位置<code>c</code>开始删除两个成员。</p> <p>如果只是单纯地插入元素，<code>splice</code>方法的第二个参数可以设为<code>0</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    768. 

  768. 

  769. a<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// [] // 不删除成员，并加入新成员到指定位置</span>
    770. 

  770. a <span class="token comment">// [1, 'a', 1, 1]</span>
    771. 

  771. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h4 id="应用-拆分数组"><a href="#应用-拆分数组" class="header-anchor">#</a> 应用-拆分数组</h4> <p>如果只提供第一个参数，等同于将原数组在指定位置<strong>拆分成两个数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    772. 

  772. <span class="token keyword">var</span> a2 <span class="token operator">=</span> a<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [3, 4]</span>
    773. 

  773. a <span class="token comment">// [1, 2]</span>
    774. 

  774. a2 <span class="token comment">// [3, 4]</span>
    775. 

  775. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="sort-排序"><a href="#sort-排序" class="header-anchor">#</a> sort() 排序</h3> <p><code>sort</code>方法对数组成员进行排序，默认是按照<strong>字典顺序排序</strong>。排序后，将<strong>改变原数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token string">'d'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    776. 

  776. <span class="token comment">// ['a', 'b', 'c', 'd']</span>
    777. 

  777. 

  778. 

  779. <span class="token punctuation">[</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    780. 

  780. <span class="token comment">// [1, 2, 3, 4]</span>
    781. 

  781. 

  782. <span class="token punctuation">[</span><span class="token string">'abc'</span><span class="token punctuation">,</span><span class="token string">'ba'</span><span class="token punctuation">,</span><span class="token string">'ea'</span><span class="token punctuation">,</span><span class="token string">'ae'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    783. 

  783. <span class="token comment">// [&quot;abc&quot;, &quot;ae&quot;, &quot;ba&quot;, &quot;ea&quot;]</span>
    784. 

  784. 

  785. <span class="token punctuation">[</span><span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">101</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    786. 

  786. <span class="token comment">// [101, 11]</span>
    787. 

  787. 

  788. <span class="token punctuation">[</span><span class="token number">10111</span><span class="token punctuation">,</span> <span class="token number">1101</span><span class="token punctuation">,</span> <span class="token number">111</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    789. 

  789. <span class="token comment">// [10111, 1101, 111]</span>
    790. 

  790. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码的最后三个例子，需要特殊注意。<code>sort()</code>方法不是按照大小排序，而是<strong>按照字典顺序</strong>。也就是说，数值会被先转成字符串，再按照字典顺序进行比较，所以<code>101</code>排在<code>11</code>的前面。</p> <blockquote><p>先转成字符串，再从0位置开始对比对应位置上的值的顺序</p></blockquote> <p>如果想让<code>sort</code>方法按照自定义方式排序，可以传入一个函数作为参数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">10111</span><span class="token punctuation">,</span> <span class="token number">1101</span><span class="token punctuation">,</span> <span class="token number">111</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 返回值大于0,则a排于b之后，否则反之。</span>
    791. 

  791.   <span class="token keyword">return</span> a <span class="token operator">-</span> b<span class="token punctuation">;</span> <span class="token comment">// 升序</span>
    792. 

  792. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    793. 

  793. <span class="token comment">// [111, 1101, 10111]</span>
    794. 

  794. 

  795. <span class="token punctuation">[</span><span class="token number">10111</span><span class="token punctuation">,</span> <span class="token number">1101</span><span class="token punctuation">,</span> <span class="token number">111</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    796. 

  796.   <span class="token keyword">return</span> b <span class="token operator">-</span> a<span class="token punctuation">;</span> <span class="token comment">// 降序</span>
    797. 

  797. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    798. 

  798. <span class="token comment">// [10111，1101, 111]</span>
    799. 

  799. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>sort</code>的参数函数本身接受两个参数，表示进行比较的两个数组成员。如果该函数的返回值大于<code>0</code>，表示第一个成员排在第二个成员后面；其他情况下，都是第一个元素排在第二个元素前面。</p> <h4 id="应用-自定义排序"><a href="#应用-自定义排序" class="header-anchor">#</a> 应用-自定义排序</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span>
    800. 

  800.   <span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">30</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    801. 

  801.   <span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;李四&quot;</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">24</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    802. 

  802.   <span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;王五&quot;</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">28</span>  <span class="token punctuation">}</span>
    803. 

  803. <span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">o1<span class="token punctuation">,</span> o2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    804. 

  804.   <span class="token keyword">return</span> o1<span class="token punctuation">.</span>age <span class="token operator">-</span> o2<span class="token punctuation">.</span>age<span class="token punctuation">;</span>
    805. 

  805. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    806. 

  806. <span class="token comment">// [</span>
    807. 

  807. <span class="token comment">//   { name: &quot;李四&quot;, age: 24 },</span>
    808. 

  808. <span class="token comment">//   { name: &quot;王五&quot;, age: 28  },</span>
    809. 

  809. <span class="token comment">//   { name: &quot;张三&quot;, age: 30 }</span>
    810. 

  810. <span class="token comment">// ]</span>
    811. 

  811. 

  812. 

  813. <span class="token comment">// 自定义排序</span>
    814. 

  814. <span class="token punctuation">[</span>
    815. 

  815.     <span class="token punctuation">{</span><span class="token literal-property property">day</span><span class="token operator">:</span> <span class="token string">'星期日'</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">day</span><span class="token operator">:</span><span class="token string">'星期一'</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">day</span><span class="token operator">:</span><span class="token string">'星期五'</span><span class="token punctuation">}</span>
    816. 

  816. <span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">o1<span class="token punctuation">,</span>o2</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    817. 

  817.     <span class="token keyword">return</span> <span class="token function">computedDay</span><span class="token punctuation">(</span>o1<span class="token punctuation">.</span>day<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token function">computedDay</span><span class="token punctuation">(</span>o2<span class="token punctuation">.</span>day<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    818. 

  818. <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">//[{ day: '星期一' }, { day: '星期五' }, { day: '星期日' }]</span>
    819. 

  819. 

  820. <span class="token keyword">function</span> <span class="token function">computedDay</span><span class="token punctuation">(</span><span class="token parameter">v</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    821. 

  821.    <span class="token keyword">return</span> v <span class="token operator">===</span> <span class="token string">'一'</span> <span class="token operator">?</span> <span class="token number">1</span> <span class="token operator">:</span> v <span class="token operator">===</span> <span class="token string">'二'</span> <span class="token operator">?</span> <span class="token number">2</span> <span class="token operator">:</span> v <span class="token operator">===</span> <span class="token string">'三'</span> <span class="token operator">?</span> <span class="token number">3</span> <span class="token operator">:</span> v <span class="token operator">===</span> <span class="token string">'四'</span> <span class="token operator">?</span> <span class="token number">4</span> <span class="token operator">:</span> v <span class="token operator">===</span> <span class="token string">'五'</span> <span class="token operator">?</span> <span class="token number">5</span> <span class="token operator">:</span> v <span class="token operator">===</span> <span class="token string">'六'</span> <span class="token operator">?</span> <span class="token number">6</span> <span class="token operator">:</span> <span class="token number">7</span>
    822. 

  822. <span class="token punctuation">}</span>
    823. 

  823. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br></div></div><p>注意，<strong>自定义的排序函数应该返回数值</strong>，否则不同的浏览器可能有不同的实现，不能保证结果都一致。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// bad</span>
    824. 

  824. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> a <span class="token operator">&gt;</span> b<span class="token punctuation">)</span> <span class="token comment">// 返回布尔值，不推荐</span>
    825. 

  825. 

  826. <span class="token comment">// good</span>
    827. 

  827. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> a <span class="token operator">-</span> b<span class="token punctuation">)</span> <span class="token comment">// 返回数值，推荐</span>
    828. 

  828. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，前一种排序算法返回的是布尔值，这是不推荐使用的。后一种是数值，才是更好的写法。</p> <h3 id="map-映射-每个成员-并返回"><a href="#map-映射-每个成员-并返回" class="header-anchor">#</a> map() 映射 每个成员 并返回</h3> <p><code>map</code>方法将数组的所有成员依次传入参数函数，然后把每一次的执行结果组成一个新数组返回。<strong>不改变原数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> numbers <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    829. 

  829. 

  830. numbers<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    831. 

  831.   <span class="token keyword">return</span> n <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
    832. 

  832. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    833. 

  833. <span class="token comment">// [2, 3, 4]</span>
    834. 

  834. 

  835. numbers
    836. 

  836. <span class="token comment">// [1, 2, 3]</span>
    837. 

  837. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>numbers</code>数组的所有成员依次执行参数函数，运行结果组成一个新数组返回，原数组没有变化。</p> <p><code>map</code>方法接受一个函数作为参数。该函数调用时，<code>map</code>方法向它传入三个参数：<strong>当前成员</strong>、<strong>当前索引</strong>、<strong>数组本身</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span> index<span class="token punctuation">,</span> arr</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 当前成员、当前索引、数组本身</span>
    838. 

  838.   <span class="token keyword">return</span> elem <span class="token operator">*</span> index<span class="token punctuation">;</span>
    839. 

  839. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    840. 

  840. <span class="token comment">// [0, 2, 6]</span>
    841. 

  841. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>map</code>方法的回调函数有三个参数，<code>elem</code>为当前成员的值，<code>index</code>为当前成员的索引，<code>arr</code>为原数组（<code>[1, 2, 3]</code>）。</p> <p><code>map</code>方法还可以接受第二个参数，用来绑定回调函数内部的<code>this</code>变量（详见《this 变量》一章）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    842. 

  842. 

  843. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    844. 

  844.   <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">[</span>e<span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// this指向arr</span>
    845. 

  845. <span class="token punctuation">}</span><span class="token punctuation">,</span> arr<span class="token punctuation">)</span> <span class="token comment">// 使内部this指向arr</span>
    846. 

  846. <span class="token comment">// ['b', 'c']</span>
    847. 

  847. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码通过<code>map</code>方法的第二个参数，将回调函数内部的<code>this</code>对象，指向<code>arr</code>数组。</p> <p>如果数组有空位，<code>map</code>方法的回调函数在这个位置不会执行，会跳过数组的空位。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token function-variable function">f</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token string">'a'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    848. 

  848. 

  849. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>f<span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;a&quot;, &quot;a&quot;]</span>
    850. 

  850. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>f<span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;a&quot;, &quot;a&quot;]</span>
    851. 

  851. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>f<span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, , &quot;a&quot;]</span>
    852. 

  852. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>map</code>方法不会跳过<code>undefined</code>和<code>null</code>，但是会跳过空位。</p> <h3 id="foreach-循环每个成员"><a href="#foreach-循环每个成员" class="header-anchor">#</a> forEach() 循环每个成员</h3> <p><code>forEach</code>方法与<code>map</code>方法很相似，也是对数组的所有成员依次执行参数函数。但是，<code>forEach</code>方法<strong>不返回值</strong>，<strong>只用来操作数据</strong>。这就是说，如果数组遍历的目的是为了得到返回值，那么使用<code>map</code>方法，否则使用<code>forEach</code>方法。</p> <p><code>forEach</code>的用法与<code>map</code>方法一致，参数是一个函数，该函数同样接受三个参数：<strong>当前成员、当前索引、整个数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">log</span><span class="token punctuation">(</span><span class="token parameter">element<span class="token punctuation">,</span> index<span class="token punctuation">,</span> array</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  <span class="token comment">// 当前成员、当前索引、数组本身</span>
    853. 

  853.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'['</span> <span class="token operator">+</span> index <span class="token operator">+</span> <span class="token string">'] = '</span> <span class="token operator">+</span> element<span class="token punctuation">)</span><span class="token punctuation">;</span>
    854. 

  854. <span class="token punctuation">}</span>
    855. 

  855. 

  856. <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">9</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>log<span class="token punctuation">)</span><span class="token punctuation">;</span>
    857. 

  857. <span class="token comment">// [0] = 2</span>
    858. 

  858. <span class="token comment">// [1] = 5</span>
    859. 

  859. <span class="token comment">// [2] = 9</span>
    860. 

  860. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>forEach</code>遍历数组不是为了得到返回值，而是为了在屏幕输出内容，所以不必使用<code>map</code>方法。</p> <p><code>forEach</code>方法也可以接受第二个参数，绑定参数函数的<code>this</code>变量。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> out <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    861. 

  861. 

  862. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">elem</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    863. 

  863.   <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>elem <span class="token operator">*</span> elem<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// this指向为out</span>
    864. 

  864. <span class="token punctuation">}</span><span class="token punctuation">,</span> out<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 使内部this指向out</span>
    865. 

  865. 

  866. out <span class="token comment">// [1, 4, 9]</span>
    867. 

  867. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，空数组<code>out</code>是<code>forEach</code>方法的第二个参数，结果，回调函数内部的<code>this</code>关键字就指向<code>out</code>。</p> <p>注意，<code>forEach</code>方法<strong>无法中断执行</strong>，总是会将所有成员遍历完。如果希望符合某种条件时，就中断遍历，要使用<code>for</code>循环。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    868. 

  868. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> arr<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    869. 

  869.   <span class="token keyword">if</span> <span class="token punctuation">(</span>arr<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    870. 

  870.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>arr<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    871. 

  871. <span class="token punctuation">}</span>
    872. 

  872. <span class="token comment">// 1</span>
    873. 

  873. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，执行到数组的第二个成员时，就会中断执行。<code>forEach</code>方法做不到这一点。</p> <p><code>forEach</code>方法也会跳过数组的空位。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token function-variable function">log</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    874. 

  874.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>n <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    875. 

  875. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    876. 

  876. 

  877. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>log<span class="token punctuation">)</span>
    878. 

  878. <span class="token comment">// 2</span>
    879. 

  879. <span class="token comment">// NaN</span>
    880. 

  880. <span class="token comment">// 3</span>
    881. 

  881. 

  882. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>log<span class="token punctuation">)</span>
    883. 

  883. <span class="token comment">// 2</span>
    884. 

  884. <span class="token comment">// 1  // null+1 =&gt; 1</span>
    885. 

  885. <span class="token comment">// 3</span>
    886. 

  886. 

  887. <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>log<span class="token punctuation">)</span>
    888. 

  888. <span class="token comment">// 2</span>
    889. 

  889. <span class="token comment">// 3</span>
    890. 

  890. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，<code>forEach</code>方法不会跳过<code>undefined</code>和<code>null</code>，但会跳过空位。</p> <h3 id="filter-过滤"><a href="#filter-过滤" class="header-anchor">#</a> filter() 过滤</h3> <p><code>filter</code>方法用于过滤数组成员，满足条件的成员组成一个新数组返回。</p> <p>它的参数是一个函数，所有数组成员依次执行该函数，返回结果为<code>true</code>的成员组成一个新数组返回。</p> <p>该方法<strong>不会改变原数组</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">elem</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    891. 

  891.     <span class="token keyword">return</span> elem <span class="token operator">&gt;</span> <span class="token number">3</span>
    892. 

  892. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    893. 

  893. <span class="token comment">// [4,5]</span>
    894. 

  894. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码将大于<code>3</code>的数组成员，作为一个<strong>新数组返回</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    895. 

  895. 

  896. arr<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>Boolean<span class="token punctuation">)</span>
    897. 

  897. <span class="token comment">// [1, &quot;a&quot;]</span>
    898. 

  898. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>filter</code>方法返回数组<code>arr</code>里面所有布尔值为<code>true</code>的成员。</p> <p><code>filter</code>方法的参数函数可以接受三个参数**：当前成员，当前位置、整个数组。**</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span> index<span class="token punctuation">,</span> arr</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 当前成员，当前位置、整个数组</span>
    899. 

  899.   <span class="token keyword">return</span> index <span class="token operator">%</span> <span class="token number">2</span> <span class="token operator">===</span> <span class="token number">0</span><span class="token punctuation">;</span>
    900. 

  900. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    901. 

  901. <span class="token comment">// [1, 3, 5]</span>
    902. 

  902. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码返回偶数位置的成员组成的新数组。</p> <p><code>filter</code>方法还可以接受第二个参数，用来绑定参数函数内部的<code>this</code>变量。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token constant">MAX</span><span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    903. 

  903. <span class="token keyword">var</span> <span class="token function-variable function">myFilter</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    904. 

  904.   <span class="token keyword">if</span> <span class="token punctuation">(</span>item <span class="token operator">&gt;</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token constant">MAX</span><span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span> <span class="token comment">// this指向obj</span>
    905. 

  905. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    906. 

  906. 

  907. <span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">9</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    908. 

  908. arr<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>myFilter<span class="token punctuation">,</span> obj<span class="token punctuation">)</span> <span class="token comment">// [8, 4, 9]  // 使myFilter内部this指向obj</span>
    909. 

  909. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，过滤器<code>myFilter</code>内部有<code>this</code>变量，它可以被<code>filter</code>方法的第二个参数<code>obj</code>绑定，返回大于<code>3</code>的成员。</p> <h3 id="some-某个-every-每个"><a href="#some-某个-every-每个" class="header-anchor">#</a> some()某个，every()每个</h3> <p>这两个方法类似“<strong>断言</strong>”（assert），返回一个布尔值，表示判断数组成员是否符合某种条件。</p> <p>它们接受一个函数作为参数，所有数组成员依次执行该函数。该函数接受三个参数：<strong>当前成员、当前位置、整个数组</strong>，然后<strong>返回一个布尔值</strong>。</p> <p><code>some</code>方法是只要一个成员的返回值是<code>true</code>，则整个<code>some</code>方法的返回值就是<code>true</code>，否则返回<code>false</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span>
    910. 

  910. arr<span class="token punctuation">.</span><span class="token function">some</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span>index<span class="token punctuation">,</span>arr</span><span class="token punctuation">)</span><span class="token punctuation">{</span> <span class="token comment">// 返回值中有一个为true则此方法返回true</span>
    911. 

  911.     <span class="token keyword">return</span> elem <span class="token operator">&gt;=</span> <span class="token number">3</span>
    912. 

  912. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    913. 

  913. <span class="token comment">// true</span>
    914. 

  914. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，如果数组<code>arr</code>有一个成员大于等于3，<code>some</code>方法就返回<code>true</code>。</p> <p><code>every</code>方法是所有成员的返回值都是<code>true</code>，整个<code>every</code>方法才返回<code>true</code>，否则返回<code>false</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    915. 

  915. arr<span class="token punctuation">.</span><span class="token function">every</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">elem<span class="token punctuation">,</span> index<span class="token punctuation">,</span> arr</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 每个返回值都为true，此方法才返回true，否则返回false</span>
    916. 

  916.   <span class="token keyword">return</span> elem <span class="token operator">&gt;=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    917. 

  917. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    918. 

  918. <span class="token comment">// false</span>
    919. 

  919. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，数组<code>arr</code>并非所有成员大于等于<code>3</code>，所以返回<code>false</code>。</p> <p>注意，对于空数组，<code>some</code>方法返回<code>false</code>，<code>every</code>方法返回<code>true</code>，回调函数都不会执行。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">isEven</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x <span class="token operator">%</span> <span class="token number">2</span> <span class="token operator">===</span> <span class="token number">0</span> <span class="token punctuation">}</span> <span class="token comment">// 不会执行此函数</span>
    920. 

  920. 

  921. <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">some</span><span class="token punctuation">(</span>isEven<span class="token punctuation">)</span> <span class="token comment">// false</span>
    922. 

  922. <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">every</span><span class="token punctuation">(</span>isEven<span class="token punctuation">)</span> <span class="token comment">// true</span>
    923. 

  923. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>some</code>和<code>every</code>方法还可以接受第二个参数，用来绑定参数函数内部的<code>this</code>变量。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span>
    924. 

  924. <span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token constant">MAX</span><span class="token operator">:</span><span class="token number">0</span><span class="token punctuation">}</span>
    925. 

  925. <span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token parameter">elem</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    926. 

  926.     <span class="token keyword">return</span> elem <span class="token operator">&gt;</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token constant">MAX</span> <span class="token comment">// this指向obj</span>
    927. 

  927. <span class="token punctuation">}</span>
    928. 

  928. 

  929. arr<span class="token punctuation">.</span><span class="token function">some</span><span class="token punctuation">(</span>fn<span class="token punctuation">,</span>obj<span class="token punctuation">)</span> <span class="token comment">// true</span>
    930. 

  930. arr<span class="token punctuation">.</span><span class="token function">every</span><span class="token punctuation">(</span>fn<span class="token punctuation">,</span>obj<span class="token punctuation">)</span> <span class="token comment">// true</span>
    931. 

  931. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="reduce-累计器-reduceright-反向累计器"><a href="#reduce-累计器-reduceright-反向累计器" class="header-anchor">#</a> reduce() 累计器，reduceRight() 反向累计器</h3> <p><code>reduce</code>方法和<code>reduceRight</code>方法依次处理数组的每个成员，最终累计为一个值。它们的差别是，<code>reduce</code>是从左到右处理（从第一个成员到最后一个成员），<code>reduceRight</code>则是从右到左（从最后一个成员到第一个成员），其他完全一样。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span>b<span class="token punctuation">,</span>c<span class="token punctuation">,</span>d</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    932. 

  932.     <span class="token comment">// 记住a是默认是第一个成员，然后是上一次运算的返回值。</span>
    933. 

  933.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span>b<span class="token punctuation">)</span>
    934. 

  934.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span>
    935. 

  935.     <span class="token keyword">return</span> a<span class="token operator">+</span>b
    936. 

  936. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    937. 

  937. <span class="token comment">// 1 2</span>
    938. 

  938. <span class="token comment">// 3 3</span>
    939. 

  939. <span class="token comment">// 6 4</span>
    940. 

  940. <span class="token comment">// 10 5</span>
    941. 

  941. <span class="token comment">// 最终结果 15</span>
    942. 

  942. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>reduce</code>方法求出数组所有成员的和。第一次执行，<code>a</code>是数组的第一个成员<code>1</code>，<code>b</code>是数组的第二个成员<code>2</code>。第二次执行，<code>a</code>为上一轮的返回值<code>3</code>，<code>b</code>为第三个成员<code>3</code>。第三次执行，<code>a</code>为上一轮的返回值<code>6</code>，<code>b</code>为第四个成员<code>4</code>。第四次执行，<code>a</code>为上一轮返回值<code>10</code>，<code>b</code>为第五个成员<code>5</code>。至此所有成员遍历完成，整个方法的返回值就是最后一轮的返回值<code>15</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>arr<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token function">callback</span><span class="token punctuation">(</span>accumulator<span class="token punctuation">,</span> currentValue<span class="token punctuation">[</span><span class="token punctuation">,</span> index<span class="token punctuation">[</span><span class="token punctuation">,</span> array<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token punctuation">,</span> initialValue<span class="token punctuation">]</span><span class="token punctuation">)</span>
    943. 

  943. <span class="token comment">// 说明：中括号代表可选参数</span>
    944. 

  944. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>reduce</code>方法和<code>reduceRight</code>方法的第一个参数都是一个<code>callback</code>函数。该函数接受以下四个参数。</p> <ol><li>累积变量，默认为数组的第一个成员</li> <li>当前变量，默认为数组的第二个成员</li> <li>当前位置（默认从1开始）, 如果提供了<code>initialValue</code>，则起始索引号为0，否则为1。 （可选）</li> <li>原数组（可选）</li></ol> <p><code>initialValue</code>可选</p> <p>作为第一次调用 <code>callback函数时</code>的第一个参数的值。 如果没有提供初始值，则将使用数组中的第一个元素。 在没有初始值的空数组上调用 reduce 将报错。</p> <p>如果要对累积变量指定初值，可以把它放在<code>reduce</code>方法和<code>reduceRight</code>方法的第二个参数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    945. 

  945.   <span class="token keyword">return</span> a <span class="token operator">+</span> b<span class="token punctuation">;</span>
    946. 

  946. <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    947. 

  947. <span class="token comment">// 25</span>
    948. 

  948. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码指定参数<code>a</code>的初值为10，所以数组从10开始累加，最终结果为25。注意，这时<code>b</code>是从数组的第一个成员开始遍历。</p> <p>上面的第二个参数相当于设定了默认值，处理空数组时尤其有用。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token parameter">prev<span class="token punctuation">,</span> cur</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    949. 

  949.   <span class="token keyword">return</span> prev <span class="token operator">+</span> cur<span class="token punctuation">;</span>
    950. 

  950. <span class="token punctuation">}</span>
    951. 

  951. 

  952. <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span>add<span class="token punctuation">)</span> <span class="token comment">// 报错</span>
    953. 

  953. <span class="token comment">// TypeError: Reduce of empty array with no initial value</span>
    954. 

  954. <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span>add<span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span>
    955. 

  955. <span class="token comment">// 1</span>
    956. 

  956. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，由于空数组取不到初始值，<code>reduce</code>方法会报错。这时，加上第二个参数，就能保证总是会返回一个值。</p> <p>下面是一个<code>reduceRight</code>方法的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">subtract</span><span class="token punctuation">(</span><span class="token parameter">prev<span class="token punctuation">,</span> cur</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    957. 

  957.   <span class="token keyword">return</span> prev <span class="token operator">-</span> cur<span class="token punctuation">;</span>
    958. 

  958. <span class="token punctuation">}</span>
    959. 

  959. 

  960. <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span>subtract<span class="token punctuation">)</span> <span class="token comment">// 0</span>
    961. 

  961. <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">reduceRight</span><span class="token punctuation">(</span>subtract<span class="token punctuation">)</span> <span class="token comment">// -4</span>
    962. 

  962. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>reduce</code>方法相当于<code>3</code>减去<code>2</code>再减去<code>1</code>，<code>reduceRight</code>方法相当于<code>1</code>减去<code>2</code>再减去<code>3</code>。</p> <h4 id="应用-找出字符最长的数组成员"><a href="#应用-找出字符最长的数组成员" class="header-anchor">#</a> 应用-找出字符最长的数组成员</h4> <p>由于这两个方法会遍历数组，所以实际上还可以用来做一些遍历相关的操作。比如，找出字符长度最长的数组成员。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>function findLongest(entries) {
    963. 

  963.   return entries.reduce(function (longest, entry) {
    964. 

  964.     return entry.length &gt; longest.length ? entry : longest;
    965. 

  965.   }, '');
    966. 

  966. }
    967. 

  967. 

  968. findLongest(['aaa', 'bb', 'c']) // &quot;aaa&quot;
    969. 

  969. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>reduce</code>的参数函数会将字符长度较长的那个数组成员，作为累积值。这导致遍历所有成员之后，累积值就是字符长度最长的那个成员。</p> <h4 id="应用-按属性对object分类"><a href="#应用-按属性对object分类" class="header-anchor">#</a> 应用-按属性对object分类</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> people <span class="token operator">=</span> <span class="token punctuation">[</span>
    970. 

  970.   <span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'Alice'</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">21</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    971. 

  971.   <span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'Max'</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">20</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    972. 

  972.   <span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'Jane'</span><span class="token punctuation">,</span> <span class="token literal-property property">age</span><span class="token operator">:</span> <span class="token number">20</span> <span class="token punctuation">}</span>
    973. 

  973. <span class="token punctuation">]</span><span class="token punctuation">;</span>
    974. 

  974. 

  975. <span class="token keyword">function</span> <span class="token function">groupBy</span><span class="token punctuation">(</span><span class="token parameter">objectArray<span class="token punctuation">,</span> property</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    976. 

  976.   <span class="token keyword">return</span> objectArray<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">acc<span class="token punctuation">,</span> obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    977. 

  977.     <span class="token keyword">var</span> key <span class="token operator">=</span> obj<span class="token punctuation">[</span>property<span class="token punctuation">]</span><span class="token punctuation">;</span>
    978. 

  978.     <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>acc<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token comment">// 当{}中没有对应分类属性时，先创建改属性</span>
    979. 

  979.       acc<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    980. 

  980.     <span class="token punctuation">}</span>
    981. 

  981.     acc<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
    982. 

  982.     <span class="token keyword">return</span> acc<span class="token punctuation">;</span>
    983. 

  983.   <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 初始值为{}</span>
    984. 

  984. <span class="token punctuation">}</span>
    985. 

  985. 

  986. <span class="token keyword">var</span> groupedPeople <span class="token operator">=</span> <span class="token function">groupBy</span><span class="token punctuation">(</span>people<span class="token punctuation">,</span> <span class="token string">'age'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    987. 

  987. <span class="token comment">// groupedPeople is:</span>
    988. 

  988. <span class="token comment">// {</span>
    989. 

  989. <span class="token comment">//   20: [</span>
    990. 

  990. <span class="token comment">//     { name: 'Max', age: 20 },</span>
    991. 

  991. <span class="token comment">//     { name: 'Jane', age: 20 }</span>
    992. 

  992. <span class="token comment">//   ],</span>
    993. 

  993. <span class="token comment">//   21: [{ name: 'Alice', age: 21 }]</span>
    994. 

  994. <span class="token comment">// }</span>
    995. 

  995. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br></div></div><p>更多例子：&lt; https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce &gt;</p> <h3 id="indexof-搜索成员第一个出现的位置-lastindexof-搜索成员最后出现的位置"><a href="#indexof-搜索成员第一个出现的位置-lastindexof-搜索成员最后出现的位置" class="header-anchor">#</a> indexOf() 搜索成员第一个出现的位置，lastIndexOf() 搜索成员最后出现的位置</h3> <p><code>indexOf</code>方法返回<strong>给定元素在数组中第一次出现的位置</strong>，如果<strong>没有则返回<code>-1</code></strong>。</p> <p><strong>indexOf语法</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>arr<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span>searchElement<span class="token punctuation">[</span><span class="token punctuation">,</span> fromIndex <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    996. 

  996. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span>
    997. 

  997. a<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// 2   3在数组a中第一次出现的位置为2</span>
    998. 

  998. a<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// -1  6在数组a中没有出现，返回-1</span>
    999. 

  999. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>indexOf</code>方法还可以接受第二个参数，表示搜索的开始位置。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// -1 从1位置开始搜索a，没有搜索到</span>
    1000. 

  1000. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码从1号位置开始搜索字符<code>a</code>，结果为<code>-1</code>，表示没有搜索到。</p> <p><strong>lastIndexOf()语法</strong></p> <div class="language- line-numbers-mode"><pre class="language-text"><code>arr.lastIndexOf(searchElement[, fromIndex = arr.length - 1])
    1001. 

  1001. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>lastIndexOf</code>方法返回<strong>给定元素在数组中最后一次出现的位置</strong>，如果没有则返回<code>-1</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">]</span>
    1002. 

  1002. a<span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">//3   2在数组a中最后一次出现的位置为3</span>
    1003. 

  1003. a<span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">//-1</span>
    1004. 

  1004. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>注意，这两个方法不能用来搜索<code>NaN</code>的位置，即它们无法确定数组成员是否包含<code>NaN</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1005. 

  1005. <span class="token punctuation">[</span><span class="token number">NaN</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1006. 

  1006. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这是因为这两个方法内部，使用严格相等运算符（<code>===</code>）进行比较，而<code>NaN</code>是唯一一个不等于自身的值。</p> <h3 id="链式使用"><a href="#链式使用" class="header-anchor">#</a> 链式使用</h3> <p>上面这些数组方法之中，有不少返回的还是数组，所以可以链式使用。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> users <span class="token operator">=</span> <span class="token punctuation">[</span>
    1007. 

  1007.   <span class="token punctuation">{</span><span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'tom'</span><span class="token punctuation">,</span> <span class="token literal-property property">email</span><span class="token operator">:</span> <span class="token string">'tom@example.com'</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
    1008. 

  1008.   <span class="token punctuation">{</span><span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">'peter'</span><span class="token punctuation">,</span> <span class="token literal-property property">email</span><span class="token operator">:</span> <span class="token string">'peter@example.com'</span><span class="token punctuation">}</span>
    1009. 

  1009. <span class="token punctuation">]</span><span class="token punctuation">;</span>
    1010. 

  1010. 

  1011. users
    1012. 

  1012. <span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">user</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1013. 

  1013.   <span class="token keyword">return</span> user<span class="token punctuation">.</span>email<span class="token punctuation">;</span>
    1014. 

  1014. <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 对每个成员进行遍历，使用返回值组成一个新的数组并返回</span>
    1015. 

  1015. <span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">email</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 过滤返回值为false的成员，并返回一个过滤掉的新数组</span>
    1016. 

  1016.   <span class="token keyword">return</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">^t</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>email<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1017. 

  1017. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1018. 

  1018. <span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">email</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 遍历数组</span>
    1019. 

  1019.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>email<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1020. 

  1020. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1021. 

  1021. <span class="token comment">// &quot;tom@example.com&quot;</span>
    1022. 

  1022. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，先产生一个所有 Email 地址组成的数组，然后再过滤出以<code>t</code>开头的 Email 地址，最后将它打印出来。</p> <blockquote><p>方法的返回值能满足下一个方法的调用，即可使用链式调用。</p></blockquote> <h2 id="四、包装对象"><a href="#四、包装对象" class="header-anchor">#</a> 四、包装对象</h2> <h2 id="_1、定义"><a href="#_1、定义" class="header-anchor">#</a> 1、定义</h2> <p>对象是JavaScript语言中最主要的数据类型，三种原始类型的值——数字、字符串、布尔值 在一定条件下，也会自动转为对象，也就是原始类型的<strong>包装对象</strong>（wrapper）</p> <p>所谓“包装对象”，<strong>指的是与数字、字符串、布尔值分别相对应的<code>Number</code>、<code>String</code>、<code>Boolean</code>三个原生对象。这三个原生对象可以把原始类型的值变成（包装成）对象</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> v1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Number</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1023. 

  1023. <span class="token keyword">var</span> v2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1024. 

  1024. <span class="token keyword">var</span> v3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1025. 

  1025. 

  1026. <span class="token keyword">typeof</span> v1 <span class="token comment">// &quot;object&quot;</span>
    1027. 

  1027. <span class="token keyword">typeof</span> v2 <span class="token comment">// &quot;object&quot;</span>
    1028. 

  1028. <span class="token keyword">typeof</span> v3 <span class="token comment">// &quot;object&quot;</span>
    1029. 

  1029. 

  1030. v1 <span class="token operator">===</span> <span class="token number">123</span> <span class="token comment">// false</span>
    1031. 

  1031. v2 <span class="token operator">===</span> <span class="token string">'abc'</span> <span class="token comment">// false</span>
    1032. 

  1032. v3 <span class="token operator">===</span> <span class="token boolean">true</span> <span class="token comment">// false</span>
    1033. 

  1033. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，基于原始类型的值，生成了三个对应的包装对象。可以看到，<code>v1</code>、<code>v2</code>、<code>v3</code>都是对象，且与对应的简单类型值不相等。</p> <p><strong>包装对象的设计目的，首先是使得“对象”这种类型可以覆盖 JavaScript 所有的值，整门语言有一个通用的数据模型，其次是使得原始类型的值也有办法调用自己的方法。</strong></p> <p><code>Number</code>、<code>String</code>和<code>Boolean</code>这三个原生对象，如果不作为构造函数调用（即调用时不加<code>new</code>），而是作为普通函数调用，常常用于将任意类型的值转为数值、字符串和布尔值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 字符串转为数值</span>
    1034. 

  1034. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'123'</span><span class="token punctuation">)</span> <span class="token comment">// 123</span>
    1035. 

  1035. 

  1036. <span class="token comment">// 数值转为字符串</span>
    1037. 

  1037. <span class="token function">String</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span> <span class="token comment">// &quot;123&quot;</span>
    1038. 

  1038. 

  1039. <span class="token comment">// 数值转为布尔值</span>
    1040. 

  1040. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1041. 

  1041. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面这种<strong>数据类型的转换</strong>，详见《数据类型转换》一节。</p> <p>总结一下，这三个对象作为构造函数使用（带有<code>new</code>）时，可以将原始类型的值转为对象；作为普通函数使用时（不带有<code>new</code>），可以将任意类型的值，转为原始类型的值。</p> <h2 id="_2、实例方法"><a href="#_2、实例方法" class="header-anchor">#</a> 2、实例方法</h2> <p>三种包装对象各自提供了许多实例方法，详见后文。这里介绍两种它们共同具有、从<code>Object</code>对象<strong>继承的方法</strong>：<code>valueOf()</code>和<code>toString()</code>。</p> <h3 id="_2-1-valueof"><a href="#_2-1-valueof" class="header-anchor">#</a> 2.1 valueOf()</h3> <p><code>valueOf()</code>方法返回包装对象实例对应的原始类型的值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Number</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// 123</span>
    1042. 

  1042. <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;abc&quot;</span>
    1043. 

  1043. <span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1044. 

  1044. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="_2-2-tostring"><a href="#_2-2-tostring" class="header-anchor">#</a> 2.2 toString()</h3> <p><code>toString()</code>方法返回对应的字符串形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Number</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;123&quot;</span>
    1045. 

  1045. <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;abc&quot;</span>
    1046. 

  1046. <span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;true&quot;</span>
    1047. 

  1047. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="_3、原始类型与实例对象的自动转换"><a href="#_3、原始类型与实例对象的自动转换" class="header-anchor">#</a> 3、原始类型与实例对象的自动转换</h2> <p>某些场合，原始类型的值会自动当作包装对象调用，即调用包装对象的属性和方法。这时，JavaScript 引擎会自动将原始类型的值转为包装对象实例，并在使用后立刻销毁实例。</p> <p>比如，字符串可以调用<code>length</code>属性，返回字符串的长度。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span>length <span class="token comment">// 3</span>
    1048. 

  1048. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>abc</code>是一个字符串，本身不是对象，不能调用<code>length</code>属性。JavaScript 引擎自动将其转为包装对象，在这个对象上调用<code>length</code>属性。调用结束后，这个临时对象就会被销毁。这就叫原始类型与实例对象的自动转换。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span>
    1049. 

  1049. str<span class="token punctuation">.</span>length <span class="token comment">// 3</span>
    1050. 

  1050. 

  1051. <span class="token comment">//等同于</span>
    1052. 

  1052. <span class="token keyword">var</span> strObj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span>
    1053. 

  1053. <span class="token comment">// String{</span>
    1054. 

  1054. <span class="token comment">//		0:'a',1:'b',2:'c',length:3,[[PrimitiveValue]]: &quot;abc&quot;</span>
    1055. 

  1055. <span class="token comment">// }</span>
    1056. 

  1056. strObj<span class="token punctuation">.</span>length <span class="token comment">// 3</span>
    1057. 

  1057. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，字符串<code>abc</code>的包装对象提供了多个属性，<code>length</code>只是其中之一。</p> <p>自动转换生成的包装对象是只读的，无法修改。所以，字符串无法添加新属性。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'Hello World'</span><span class="token punctuation">;</span>
    1058. 

  1058. s<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
    1059. 

  1059. s<span class="token punctuation">.</span>x <span class="token comment">// undefined</span>
    1060. 

  1060. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码为字符串<code>s</code>添加了一个<code>x</code>属性，结果无效，总是返回<code>undefined</code>。</p> <p>另一方面，调用结束后，包装对象实例会自动销毁。这意味着，下一次调用字符串的属性时，实际是调用一个新生成的对象，而不是上一次调用时生成的那个对象，所以取不到赋值在上一个对象的属性。如果要为字符串添加属性，只有在它的原型对象<code>String.prototype</code>上定义（参见《面向对象编程》章节）。</p> <h2 id="_4、自定义方法"><a href="#_4、自定义方法" class="header-anchor">#</a> 4、自定义方法</h2> <p>除了原生的实例方法，包装对象还可以自定义方法和属性，供原始类型的值直接调用。</p> <p>比如，我们可以新增一个<code>double</code>方法，使得字符串和数字翻倍。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">String</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">double</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// String是一个方法，String.prototype本质是一个实例对象</span>
    1061. 

  1061.   <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1062. 

  1062. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    1063. 

  1063. 

  1064. <span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">double</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1065. 

  1065. <span class="token comment">// abcabc</span>
    1066. 

  1066. 

  1067. <span class="token class-name">Number</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">double</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1068. 

  1068.   <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1069. 

  1069. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    1070. 

  1070. 

  1071. <span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">double</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 246 // 不加括号时点运算符会被解释成小数点</span>
    1072. 

  1072. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码在<code>String</code>和<code>Number</code>这两个对象的原型上面，分别自定义了一个方法，从而可以在所有实例对象上调用。注意，最后一张的<code>123</code>外面必须要加上圆括号，否则后面的点运算符（<code>.</code>）会被解释成小数点。</p> <h2 id="五、boolean对象"><a href="#五、boolean对象" class="header-anchor">#</a> 五、Boolean对象</h2> <h2 id="_1、概述-3"><a href="#_1、概述-3" class="header-anchor">#</a> 1、概述</h2> <p><code>Boolean</code>对象是 JavaScript 的三个包装对象之一。作为构造函数，它主要用于生成布尔值的包装对象实例。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span>
    1073. 

  1073. <span class="token keyword">typeof</span> b <span class="token comment">// object</span>
    1074. 

  1074. b<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1075. 

  1075. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码的变量<code>b</code>是一个<code>Boolean</code>对象的实例，它的类型是对象，值为布尔值<code>true</code>。</p> <p>注意，<code>false</code>对应的包装对象实例，布尔运算结果也是<code>true</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 返回的是一个对象，所有对象对应的布尔值都是true</span>
    1076. 

  1076.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1077. 

  1077. <span class="token punctuation">}</span> <span class="token comment">// true</span>
    1078. 

  1078. 

  1079. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 它的值为false</span>
    1080. 

  1080.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1081. 

  1081. <span class="token punctuation">}</span> <span class="token comment">// 无输出</span>
    1082. 

  1082. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码的第一个例子之所以得到<code>true</code>，是因为<code>false</code>对应的包装对象实例是一个对象，进行逻辑运算时，被自动转化成布尔值<code>true</code>（因为所有对象对应的布尔值都是<code>true</code>）。而实例的<code>valueOf</code>方法，则返回实例对应的原始值，本例为<code>false</code>。</p> <h2 id="_2、boolean-函数的类型转换作用"><a href="#_2、boolean-函数的类型转换作用" class="header-anchor">#</a> 2、Boolean 函数的类型转换作用</h2> <p><code>Boolean</code>对象除了可以作为构造函数，还可以单独使用，将任意值转为布尔值。这时<code>Boolean</code>就是一个<strong>单纯的工具方法</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1083. 

  1083. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1084. 

  1084. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1085. 

  1085. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1086. 

  1086. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1087. 

  1087. 

  1088. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1089. 

  1089. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token string">'false'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1090. 

  1090. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1091. 

  1091. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1092. 

  1092. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1093. 

  1093. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">foo</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1094. 

  1094. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中几种得到<code>true</code>的情况，<strong>都值得认真记住</strong>。</p> <blockquote><p>直接使用上面方法括号内的参数进行if逻辑运行相当于调用了Boolean方法并传入参数</p></blockquote> <p>顺便提一下，使用双重的否运算符（<code>!</code>）也可以将任意值转为对应的布尔值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token operator">!</span><span class="token operator">!</span><span class="token keyword">undefined</span> <span class="token comment">// false</span>
    1095. 

  1095. <span class="token operator">!</span><span class="token operator">!</span><span class="token keyword">null</span> <span class="token comment">// false</span>
    1096. 

  1096. <span class="token operator">!</span><span class="token operator">!</span><span class="token number">0</span> <span class="token comment">// false</span>
    1097. 

  1097. <span class="token operator">!</span><span class="token operator">!</span><span class="token string">''</span> <span class="token comment">// false</span>
    1098. 

  1098. <span class="token operator">!</span><span class="token operator">!</span><span class="token number">NaN</span> <span class="token comment">// false</span>
    1099. 

  1099. 

  1100. <span class="token operator">!</span><span class="token operator">!</span><span class="token number">1</span> <span class="token comment">// true</span>
    1101. 

  1101. <span class="token operator">!</span><span class="token operator">!</span><span class="token string">'false'</span> <span class="token comment">// true</span>
    1102. 

  1102. <span class="token operator">!</span><span class="token operator">!</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token comment">// true</span>
    1103. 

  1103. <span class="token operator">!</span><span class="token operator">!</span><span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token comment">// true</span>
    1104. 

  1104. <span class="token operator">!</span><span class="token operator">!</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token comment">// true</span>
    1105. 

  1105. <span class="token operator">!</span><span class="token operator">!</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">foo</span><span class="token regex-delimiter">/</span></span> <span class="token comment">// true</span>
    1106. 

  1106. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>最后，对于一些特殊值，<code>Boolean</code>对象前面加不加<code>new</code>，会得到完全相反的结果，必须小心。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1107. 

  1107.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1108. 

  1108. <span class="token punctuation">}</span> <span class="token comment">// 无输出</span>
    1109. 

  1109. 

  1110. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 加new 转为对象，对象为true</span>
    1111. 

  1111.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1112. 

  1112. <span class="token punctuation">}</span> <span class="token comment">// true</span>
    1113. 

  1113. 

  1114. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1115. 

  1115.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1116. 

  1116. <span class="token punctuation">}</span> <span class="token comment">// 无输出</span>
    1117. 

  1117. 

  1118. <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Boolean</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 加new 转为对象，对象为true</span>
    1119. 

  1119.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1120. 

  1120. <span class="token punctuation">}</span> <span class="token comment">// true</span>
    1121. 

  1121. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><h2 id="六、number对象"><a href="#六、number对象" class="header-anchor">#</a> 六、Number对象</h2> <h2 id="_1、概述-4"><a href="#_1、概述-4" class="header-anchor">#</a> 1、概述</h2> <p><code>Number</code>对象是数值对应的包装对象，可以作为构造函数使用，也可以作为工具函数使用。</p> <p>作为构造函数时，它用于生成值为数值的对象。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> n <span class="token operator">=</span><span class="token keyword">new</span> <span class="token class-name">Number</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 作为构造函数时，生成值为数值的对象</span>
    1122. 

  1122. <span class="token keyword">typeof</span> n <span class="token comment">// object</span>
    1123. 

  1123. n<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1124. 

  1124. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>Number</code>对象作为构造函数使用，返回一个值为<code>1</code>的对象。</p> <p>作为工具函数时，它可以将任何类型的值转为数值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">Number</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1125. 

  1125. 

  1126. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1127. 

  1127. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1128. 

  1128. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1129. 

  1129. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1130. 

  1130. 

  1131. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    1132. 

  1132. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    1133. 

  1133. 

  1134. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'123'</span><span class="token punctuation">)</span><span class="token comment">// 123</span>
    1135. 

  1135. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码将布尔值<code>true</code>转为数值<code>1</code>。<code>Number</code>作为工具函数的用法，详见《数据类型转换》一章。</p> <h2 id="_2、静态属性"><a href="#_2、静态属性" class="header-anchor">#</a> 2、静态属性</h2> <p><code>Number</code>对象拥有以下一些静态属性（即直接定义在<code>Number</code>对象上的属性，而不是定义在实例上的属性）。</p> <ul><li><code>Number.POSITIVE_INFINITY</code>：正的无限，指向<code>Infinity</code>。</li> <li><code>Number.NEGATIVE_INFINITY</code>：负的无限，指向<code>-Infinity</code>。</li> <li><code>Number.NaN</code>：表示非数值，指向<code>NaN</code>。</li> <li><code>Number.MAX_VALUE</code>：表示最大的正数<code>1.7976931348623157e+308</code>，相应的，最小的负数为<code>-Number.MAN_VALUE</code>。</li> <li><code>Number.MIN_VALUE</code>：表示最小的正数（即最接近0的正数，在64位浮点数体系中为<code>5e-324</code>），相应的，最接近0的负数为<code>-Number.MIN_VALUE</code>。</li> <li><code>Number.MAX_SAFE_INTEGER</code>：表示能够精确表示的最大整数，即<code>9007199254740991</code>。</li> <li><code>Number.MIN_SAFE_INTEGER</code>：表示能够精确表示的最小整数，即<code>-9007199254740991</code>。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Number<span class="token punctuation">.</span><span class="token constant">POSITIVE_INFINITY</span> <span class="token comment">// Infinity</span>
    1136. 

  1136. Number<span class="token punctuation">.</span><span class="token constant">NEGATIVE_INFINITY</span> <span class="token comment">// -Infinity</span>
    1137. 

  1137. Number<span class="token punctuation">.</span><span class="token number">NaN</span> <span class="token comment">// NaN</span>
    1138. 

  1138. 

  1139. Number<span class="token punctuation">.</span><span class="token constant">MAX_VALUE</span>
    1140. 

  1140. <span class="token comment">// 1.7976931348623157e+308</span>
    1141. 

  1141. Number<span class="token punctuation">.</span><span class="token constant">MAX_VALUE</span> <span class="token operator">&lt;</span> <span class="token number">Infinity</span>
    1142. 

  1142. <span class="token comment">// true</span>
    1143. 

  1143. 

  1144. Number<span class="token punctuation">.</span><span class="token constant">MIN_VALUE</span>
    1145. 

  1145. <span class="token comment">// 5e-324</span>
    1146. 

  1146. Number<span class="token punctuation">.</span><span class="token constant">MIN_VALUE</span> <span class="token operator">&gt;</span> <span class="token number">0</span>
    1147. 

  1147. <span class="token comment">// true</span>
    1148. 

  1148. 

  1149. Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span> <span class="token comment">// 9007199254740991</span>
    1150. 

  1150. Number<span class="token punctuation">.</span><span class="token constant">MIN_SAFE_INTEGER</span> <span class="token comment">// -9007199254740991</span>
    1151. 

  1151. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><h2 id="_3、实例方法-2"><a href="#_3、实例方法-2" class="header-anchor">#</a> 3、实例方法</h2> <p><code>Number</code>对象有<strong>4个实例方法</strong>，都跟<strong>将数值转换成指定格式有关</strong>。</p> <h3 id="_3-1-number-prototype-tostring-可转成多种进制字符串"><a href="#_3-1-number-prototype-tostring-可转成多种进制字符串" class="header-anchor">#</a> 3.1 Number.prototype.toString() 可转成多种进制字符串</h3> <p><code>Number</code>对象部署了自己的<code>toString</code>方法，用来将一个数值转为字符串形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;10&quot;</span>
    1152. 

  1152. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>toString</code>方法可以<strong>接受一个参数</strong>，表示输出的<strong>进制</strong>。如果省略这个参数，默认将数值先转为十进制，再输出字符串；否则，就根据参数指定的进制，将一个数字转化成某个进制的字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1010&quot;</span>
    1153. 

  1153. <span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">8</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12&quot;</span>
    1154. 

  1154. <span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span> <span class="token comment">// &quot;a&quot;</span>
    1155. 

  1155. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>10</code>一定要放在括号里，这样表明后面的点表示调用对象属性。如果不加括号，这个点会被 JavaScript 引擎解释成小数点，从而报错。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token number">10.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 不加括号会把点解释成小数点，从而报错</span>
    1156. 

  1156. <span class="token comment">// SyntaxError: Unexpected token ILLEGAL</span>
    1157. 

  1157. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>只要能够让 JavaScript 引擎不混淆小数点和对象的点运算符，各种写法都能用。除了为<code>10</code>加上括号，还可以在<code>10</code>后面加两个点，JavaScript 会把第一个点理解成小数点（即<code>10.0</code>），把第二个点理解成调用对象属性，从而得到正确结果。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token number">10.</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
    1158. 

  1158. <span class="token comment">// &quot;1010&quot;</span>
    1159. 

  1159. 

  1160. <span class="token comment">// 其他方法还包括</span>
    1161. 

  1161. <span class="token number">10</span> <span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1010&quot;</span>
    1162. 

  1162. <span class="token number">10.0</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1010&quot;</span>
    1163. 

  1163. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>这实际上意味着，可以直接对一个小数使用<code>toString</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token number">10.5</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;10.5&quot;</span>
    1164. 

  1164. <span class="token number">10.5</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1010.1&quot;</span>
    1165. 

  1165. <span class="token number">10.5</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">8</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.4&quot;</span>
    1166. 

  1166. <span class="token number">10.5</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span> <span class="token comment">// &quot;a.8&quot;</span>
    1167. 

  1167. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>通过方括号运算符也可以调用<code>toString</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token number">10</span><span class="token punctuation">[</span><span class="token string">'toString'</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1010&quot;</span>
    1168. 

  1168. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>toString</code>方法只能将十进制的数，转为其他进制的字符串。如果要将其他进制的数，转回十进制，需要使用<code>parseInt</code>方法。</p> <h3 id="_3-2-number-prototype-tofixed-转成保留指定小数位数的字符串"><a href="#_3-2-number-prototype-tofixed-转成保留指定小数位数的字符串" class="header-anchor">#</a> 3.2 Number.prototype.toFixed() 转成保留指定小数位数的字符串</h3> <p><code>toFixed()</code>方法先将一个数转为指定位数的小数，然后返回这个小数对应的字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toFixed</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;10.00&quot;</span>
    1169. 

  1169. <span class="token number">10.005</span><span class="token punctuation">.</span><span class="token function">toFixed</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;10.01&quot;</span>
    1170. 

  1170. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>10</code>和<code>10.005</code>先转成2位小数，然后转成字符串。其中<code>10</code>必须放在括号里，否则后面的点会被处理成小数点。</p> <p><code>toFixed()</code>方法的参数为小数位数，有效范围为0到20，超出这个范围将抛出 RangeError 错误。</p> <p>由于浮点数的原因，小数<code>5</code>的四舍五入是不确定的，使用的时候必须小心。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">10.055</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toFixed</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 10.05</span>
    1171. 

  1171. <span class="token punctuation">(</span><span class="token number">10.005</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toFixed</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 10.01</span>
    1172. 

  1172. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_3-3-number-prototype-toexponential-转成科学计数法形式字符串"><a href="#_3-3-number-prototype-toexponential-转成科学计数法形式字符串" class="header-anchor">#</a> 3.3 Number.prototype.toExponential() 转成科学计数法形式字符串</h3> <p><code>toExponential</code>方法用于将一个数转为科学计数法形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toExponential</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// &quot;1e+1&quot;</span>
    1173. 

  1173. <span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toExponential</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1.0e+1&quot;</span>
    1174. 

  1174. <span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toExponential</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1.00e+1&quot;</span>
    1175. 

  1175. 

  1176. <span class="token punctuation">(</span><span class="token number">1234</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toExponential</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// &quot;1.234e+3&quot;</span>
    1177. 

  1177. <span class="token punctuation">(</span><span class="token number">1234</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toExponential</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1.2e+3&quot;</span>
    1178. 

  1178. <span class="token punctuation">(</span><span class="token number">1234</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toExponential</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1.23e+3&quot;</span>
    1179. 

  1179. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p><code>toExponential</code>方法的参数是小数点后有效数字的位数，范围为0到20，超出这个范围，会抛出一个 RangeError 错误。</p> <h3 id="_3-4-number-prototype-toprecision-转为指定位数的有效数字"><a href="#_3-4-number-prototype-toprecision-转为指定位数的有效数字" class="header-anchor">#</a> 3.4 Number.prototype.toPrecision()转为指定位数的有效数字</h3> <p><code>Number.prototype.toPrecision()</code>方法用于将一个数转为指定位数的有效数字。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">12.34</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1e+1&quot;</span>
    1180. 

  1180. <span class="token punctuation">(</span><span class="token number">12.34</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12&quot;</span>
    1181. 

  1181. <span class="token punctuation">(</span><span class="token number">12.34</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.3&quot;</span>
    1182. 

  1182. <span class="token punctuation">(</span><span class="token number">12.34</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.34&quot;</span>
    1183. 

  1183. <span class="token punctuation">(</span><span class="token number">12.34</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.340&quot;</span>
    1184. 

  1184. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>该方法的参数为有效数字的位数，范围是1到21，超出这个范围会抛出 RangeError 错误。</p> <p>该方法用于四舍五入时不太可靠，跟浮点数不是精确储存有关。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">12.35</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.3&quot;</span>
    1185. 

  1185. <span class="token punctuation">(</span><span class="token number">12.25</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.3&quot;</span>
    1186. 

  1186. <span class="token punctuation">(</span><span class="token number">12.15</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.2&quot;</span>
    1187. 

  1187. <span class="token punctuation">(</span><span class="token number">12.45</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toPrecision</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12.4&quot;</span>
    1188. 

  1188. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="_4-5-number-prototype-tolocalestring"><a href="#_4-5-number-prototype-tolocalestring" class="header-anchor">#</a> 4.5 Number.prototype.toLocaleString()</h3> <p><code>Number.prototype.toLocaleString()</code>方法接受一个地区码作为参数，返回一个字符串，表示当前数字在该地区的当地书写形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-Hans-CN-u-nu-hanidec'</span><span class="token punctuation">)</span>
    1189. 

  1189. <span class="token comment">// &quot;一二三&quot;</span>
    1190. 

  1190. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>该方法还可以接受第二个参数配置对象，用来定制指定用途的返回字符串。该对象的<code>style</code>属性指定输出样式，默认值是<code>decimal</code>，表示输出十进制形式。如果值为<code>percent</code>，表示输出百分数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-Hans-CN'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">style</span><span class="token operator">:</span> <span class="token string">'persent'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1191. 

  1191. <span class="token comment">// &quot;12,300%&quot;</span>
    1192. 

  1192. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果<code>style</code>属性的值为<code>currency</code>，则可以搭配<code>currency</code>属性，输出指定格式的货币字符串形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-Hans-CN'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">style</span><span class="token operator">:</span> <span class="token string">'currency'</span><span class="token punctuation">,</span> <span class="token literal-property property">currency</span><span class="token operator">:</span> <span class="token string">'CNY'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1193. 

  1193. <span class="token comment">// &quot;￥123.00&quot;</span>
    1194. 

  1194. 

  1195. <span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'de-DE'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">style</span><span class="token operator">:</span> <span class="token string">'currency'</span><span class="token punctuation">,</span> <span class="token literal-property property">currency</span><span class="token operator">:</span> <span class="token string">'EUR'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1196. 

  1196. <span class="token comment">// &quot;123,00 €&quot;</span>
    1197. 

  1197. 

  1198. <span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">style</span><span class="token operator">:</span> <span class="token string">'currency'</span><span class="token punctuation">,</span> <span class="token literal-property property">currency</span><span class="token operator">:</span> <span class="token string">'USD'</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1199. 

  1199. <span class="token comment">// &quot;$123.00&quot;</span>
    1200. 

  1200. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>如果<code>Number.prototype.toLocaleString()</code>省略了参数，则由浏览器自行决定如何处理，通常会使用操作系统的地区设定。注意，该方法如果使用浏览器不认识的地区码，会抛出一个错误。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'123'</span><span class="token punctuation">)</span> <span class="token comment">// 出错</span>
    1201. 

  1201. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h4 id="应用-添加千位分隔符"><a href="#应用-添加千位分隔符" class="header-anchor">#</a> 应用-添加千位分隔符</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">123456.789</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1202. 

  1202. <span class="token comment">// 输出 &quot;123,456.789&quot;，常用。</span>
    1203. 

  1203. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token number">1234566</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-Hans-CN-u-nu-hanidec'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1204. 

  1204. <span class="token comment">// 输出&quot;一,二三四,五六六&quot;</span>
    1205. 

  1205. 

  1206. <span class="token punctuation">(</span><span class="token number">1234566</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-Hans-CN-u-nu-hanidec'</span><span class="token punctuation">,</span><span class="token punctuation">{</span><span class="token literal-property property">useGrouping</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1207. 

  1207. <span class="token comment">// 给.toLocaleString()加第二个参数，依然输出&quot;一,二三四,五六六&quot;</span>
    1208. 

  1208. 

  1209. <span class="token comment">// 不带千位分隔符</span>
    1210. 

  1210. <span class="token punctuation">(</span><span class="token number">1234566</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-Hans-CN-u-nu-hanidec'</span><span class="token punctuation">,</span><span class="token punctuation">{</span><span class="token literal-property property">useGrouping</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1211. 

  1211. <span class="token comment">// 输出&quot;一二三四五六六&quot;，这个比较有用，也就是最简单的将阿拉伯数字变成汉字的办法。</span>
    1212. 

  1212. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h2 id="_4、自定义方法-2"><a href="#_4、自定义方法-2" class="header-anchor">#</a> 4、自定义方法</h2> <p>与其他对象一样，<code>Number.prototype</code>对象上面可以自定义方法，被<code>Number</code>的实例继承。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Number</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">add</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1213. 

  1213.   <span class="token keyword">return</span> <span class="token keyword">this</span> <span class="token operator">+</span> x<span class="token punctuation">;</span>
    1214. 

  1214. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    1215. 

  1215. 

  1216. <span class="token number">8</span><span class="token punctuation">[</span><span class="token string">'add'</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 10</span>
    1217. 

  1217. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码为<code>Number</code>对象实例定义了一个<code>add</code>方法。在数值上调用某个方法，数值会自动转为<code>Number</code>的实例对象，所以就可以调用<code>add</code>方法了。由于<code>add</code>方法返回的还是数值，所以可以链式运算。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Number</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">subtract</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1218. 

  1218.   <span class="token keyword">return</span> <span class="token keyword">this</span> <span class="token operator">-</span> x<span class="token punctuation">;</span>
    1219. 

  1219. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    1220. 

  1220. 

  1221. <span class="token punctuation">(</span><span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">subtract</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span>
    1222. 

  1222. <span class="token comment">// 6</span>
    1223. 

  1223. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码在<code>Number</code>对象的实例上部署了<code>subtract</code>方法，它可以与<code>add</code>方法链式调用。</p> <p>我们还可以部署更复杂的方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Number</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">iterate</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1224. 

  1224.   <span class="token keyword">var</span> result <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    1225. 

  1225.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;=</span> <span class="token keyword">this</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1226. 

  1226.     result<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1227. 

  1227.   <span class="token punctuation">}</span>
    1228. 

  1228.   <span class="token keyword">return</span> result<span class="token punctuation">;</span>
    1229. 

  1229. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    1230. 

  1230. 

  1231. <span class="token punctuation">(</span><span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">iterate</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1232. 

  1232. <span class="token comment">// [0, 1, 2, 3, 4, 5, 6, 7, 8]</span>
    1233. 

  1233. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码在<code>Number</code>对象的原型上部署了<code>iterate</code>方法，将一个数值自动遍历为一个数组。</p> <p><strong>注意</strong>，数值的自定义方法，只能定义在它的原型对象<code>Number.prototype</code>上面，数值本身是无法自定义属性的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> n <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    1234. 

  1234. n<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    1235. 

  1235. n<span class="token punctuation">.</span>x <span class="token comment">// undefined</span>
    1236. 

  1236. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>n</code>是一个原始类型的数值。直接在它上面新增一个属性<code>x</code>，不会报错，但毫无作用，总是返回<code>undefined</code>。这是因为一旦被调用属性，<code>n</code>就自动转为<code>Number</code>的实例对象，调用结束后，该对象自动销毁。所以，下一次调用<code>n</code>的属性时，实际取到的是另一个对象，属性<code>x</code>当然就读不出来。</p> <h2 id="七、string对象"><a href="#七、string对象" class="header-anchor">#</a> 七、String对象</h2> <h2 id="_1、概述-5"><a href="#_1、概述-5" class="header-anchor">#</a> 1、概述</h2> <p><code>String</code>对象是 JavaScript 原生提供的三个包装对象之一，用来生成字符串对象。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s1 <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">;</span>
    1237. 

  1237. <span class="token keyword">var</span> s2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 使用new关键字会变成包装对象</span>
    1238. 

  1238. 

  1239. <span class="token keyword">typeof</span> s1 <span class="token comment">// &quot;string&quot;</span>
    1240. 

  1240. <span class="token keyword">typeof</span> s2 <span class="token comment">// &quot;object&quot;</span>
    1241. 

  1241. 

  1242. s2<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;abc&quot;</span>
    1243. 

  1243. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，变量<code>s1</code>是字符串，<code>s2</code>是对象。由于<code>s2</code>是字符串对象，<code>s2.valueOf</code>方法返回的就是它所对应的原始字符串。</p> <p><strong>字符串对象是一个类似数组的对象</strong>（很像数组，但不是数组）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> strObj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span>
    1244. 

  1244. <span class="token comment">// String {0: &quot;a&quot;, 1: &quot;b&quot;, 2: &quot;c&quot;, length: 3}</span>
    1245. 

  1245. 

  1246. strObj<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span> <span class="token comment">// &quot;b&quot;</span>
    1247. 

  1247. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，字符串<code>abc</code>对应的字符串对象，有数值键（<code>0</code>、<code>1</code>、<code>2</code>）和<code>length</code>属性，所以可以像数组那样取值。</p> <p>除了用作构造函数，<code>String</code>对象还可以当作工具方法使用，将任意类型的值转为字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">String</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// &quot;true&quot;</span>
    1248. 

  1248. <span class="token function">String</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token comment">// &quot;false&quot;</span>
    1249. 

  1249. <span class="token function">String</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// &quot;5&quot;</span>
    1250. 

  1250. <span class="token function">String</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// 'null'</span>
    1251. 

  1251. <span class="token function">String</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">//'undefined'</span>
    1252. 

  1252. <span class="token function">String</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// '[object Object]'</span>
    1253. 

  1253. <span class="token function">String</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// ''</span>
    1254. 

  1254. <span class="token function">String</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'1'</span><span class="token punctuation">,</span><span class="token string">'2'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// '1,2'</span>
    1255. 

  1255. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码将各种类型的值，分别转换为字符串。</p> <h2 id="_2、静态方法-2"><a href="#_2、静态方法-2" class="header-anchor">#</a> 2、静态方法</h2> <h3 id="string-fromcharcode"><a href="#string-fromcharcode" class="header-anchor">#</a> String.fromCharCode()</h3> <p><code>String</code>对象提供的静态方法（即定义在对象本身，而不是定义在对象实例的方法），主要是<code>String.fromCharCode()</code>。<strong>该方法的参数是一个或多个数值，代表 Unicode 码点，返回值是这些码点组成的字符串。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
    1256. 

  1256. String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">97</span><span class="token punctuation">)</span> <span class="token comment">// &quot;a&quot;</span>
    1257. 

  1257. String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">104</span><span class="token punctuation">,</span> <span class="token number">101</span><span class="token punctuation">,</span> <span class="token number">108</span><span class="token punctuation">,</span> <span class="token number">108</span><span class="token punctuation">,</span> <span class="token number">111</span><span class="token punctuation">)</span>
    1258. 

  1258. <span class="token comment">// &quot;hello&quot;</span>
    1259. 

  1259. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>String.fromCharCode</code>方法的参数为空，就返回空字符串；否则，返回参数对应的 Unicode 字符串。</p> <p>注意，该方法不支持 Unicode 码点大于<code>0xFFFF</code>的字符，即传入的参数不能大于<code>0xFFFF</code>（即十进制的 65535）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">0x20BB7</span><span class="token punctuation">)</span>
    1260. 

  1260. <span class="token comment">// &quot;ஷ&quot;</span>
    1261. 

  1261. String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">0x20BB7</span><span class="token punctuation">)</span> <span class="token operator">===</span> String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">0x0BB7</span><span class="token punctuation">)</span>
    1262. 

  1262. <span class="token comment">// true</span>
    1263. 

  1263. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>String.fromCharCode</code>参数<code>0x20BB7</code>大于<code>0xFFFF</code>，导致返回结果出错。<code>0x20BB7</code>对应的字符是汉字<code>𠮷</code>，但是返回结果却是另一个字符（码点<code>0x0BB7</code>）。这是因为<code>String.fromCharCode</code>发现参数值大于<code>0xFFFF</code>，就会忽略多出的位（即忽略<code>0x20BB7</code>里面的<code>2</code>）。</p> <p>这种现象的根本原因在于，码点大于<code>0xFFFF</code>的字符占用四个字节，而 JavaScript 默认支持两个字节的字符。这种情况下，必须把<code>0x20BB7</code>拆成两个字符表示。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">0xD842</span><span class="token punctuation">,</span> <span class="token number">0xDFB7</span><span class="token punctuation">)</span>
    1264. 

  1264. <span class="token comment">// &quot;𠮷&quot;</span>
    1265. 

  1265. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>0x20BB7</code>拆成两个字符<code>0xD842</code>和<code>0xDFB7</code>（即两个两字节字符，合成一个四字节字符），就能得到正确的结果。码点大于<code>0xFFFF</code>的字符的四字节表示法，由 UTF-16 编码方法决定。</p> <h2 id="_3、实例属性"><a href="#_3、实例属性" class="header-anchor">#</a> 3、实例属性</h2> <h3 id="string-prototype-length"><a href="#string-prototype-length" class="header-anchor">#</a> String.prototype.length</h3> <p>字符串实例的<code>length</code>属性返回字符串的长度。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span>length <span class="token comment">// 3</span>
    1266. 

  1266. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h2 id="_4、实例方法"><a href="#_4、实例方法" class="header-anchor">#</a> 4、实例方法</h2> <h3 id="_4-1-string-prototype-charat-返回指定位置的字符"><a href="#_4-1-string-prototype-charat-返回指定位置的字符" class="header-anchor">#</a> 4.1 String.prototype.charAt() 返回指定位置的字符</h3> <p><code>charAt</code>方法<strong>返回指定位置的字符</strong>，参数是从<code>0</code>开始编号的位置。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1267. 

  1267. 

  1268. s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;b&quot;</span>
    1269. 

  1269. s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>s<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;c&quot;</span>
    1270. 

  1270. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>这个方法完全可以用数组下标替代。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;b&quot;</span>
    1271. 

  1271. <span class="token string">'abc'</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span> <span class="token comment">// &quot;b&quot;</span>
    1272. 

  1272. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果参数为负数，或大于等于字符串的长度，<code>charAt</code>返回空字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
    1273. 

  1273. <span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
    1274. 

  1274. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_4-2-string-prototype-charcodeat-返回字符串指定位置的-unicode-码点-十进制表示"><a href="#_4-2-string-prototype-charcodeat-返回字符串指定位置的-unicode-码点-十进制表示" class="header-anchor">#</a> 4.2 String.prototype.charCodeAt()返回字符串指定位置的 Unicode 码点（十进制表示）</h3> <p><code>charCodeAt</code>方法<strong>返回字符串指定位置的 Unicode 码点（十进制表示）</strong>，相当于<code>String.fromCharCode()</code>的逆操作。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charCodeAt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 98</span>
    1275. 

  1275. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>abc</code>的<code>1</code>号位置的字符是<code>b</code>，它的 Unicode 码点是<code>98</code>。</p> <p>如果没有任何参数，<code>charCodeAt</code>返回首字符的 Unicode 码点。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charCodeAt</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 97</span>
    1276. 

  1276. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果参数为负数，或大于等于字符串的长度，<code>charCodeAt</code>返回<code>NaN</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charCodeAt</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    1277. 

  1277. <span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">charCodeAt</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    1278. 

  1278. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><strong>注意</strong>，<code>charCodeAt</code>方法返回的 Unicode 码点不会大于65536（0xFFFF），也就是说，只返回两个字节的字符的码点。如果遇到码点大于 65536 的字符（四个字节的字符），必需连续使用两次<code>charCodeAt</code>，不仅读入<code>charCodeAt(i)</code>，还要读入<code>charCodeAt(i+1)</code>，将两个值放在一起，才能得到准确的字符。</p> <h3 id="_4-3-string-prototype-concat-合并字符串-返回新字符串-不改变原字符串"><a href="#_4-3-string-prototype-concat-合并字符串-返回新字符串-不改变原字符串" class="header-anchor">#</a> 4.3 String.prototype.concat() 合并字符串，返回新字符串，不改变原字符串</h3> <p><code>concat</code>方法<strong>用于连接两个字符串，返回一个新字符串，不改变原字符串</strong>。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var s1 = 'abc';
    1279. 

  1279. var s2 = 'def';
    1280. 

  1280. 

  1281. s1.concat(s2) // &quot;abcdef&quot;
    1282. 

  1282. s1 // &quot;abc&quot;
    1283. 

  1283. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>该方法可以接受多个参数。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>'a'.concat('b', 'c') // &quot;abc&quot;
    1284. 

  1284. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果参数不是字符串，<code>concat</code>方法会将其先转为字符串，然后再连接。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var one = 1;
    1285. 

  1285. var two = 2;
    1286. 

  1286. var three = '3';
    1287. 

  1287. 

  1288. ''.concat(one, two, three) // &quot;123&quot;
    1289. 

  1289. one + two + three // &quot;33&quot;
    1290. 

  1290. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>concat</code>方法将参数先转成字符串再连接，所以返回的是一个三个字符的字符串。作为对比，加号运算符在两个运算数都是数值时，不会转换类型，所以返回的是一个两个字符的字符串。</p> <h3 id="_4-4-string-prototype-slice-取出部分子字符串-不改变原字符串"><a href="#_4-4-string-prototype-slice-取出部分子字符串-不改变原字符串" class="header-anchor">#</a> 4.4 String.prototype.slice() 取出部分子字符串，不改变原字符串</h3> <p><code>slice</code>方法<strong>用于从原字符串取出子字符串并返回，不改变原字符串</strong>。它的第一个参数是子字符串的<strong>开始位置</strong>，第二个参数是子字符串的<strong>结束位置（不含该位置）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Java&quot;</span>
    1291. 

  1291. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果省略第二个参数，则表示子字符串一直到原字符串结束。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot; 省略参数二，会取到结束</span>
    1292. 

  1292. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果参数是负值，表示从结尾开始倒数计算的位置，即该负值加上字符串长度。</p> <h5 id="负数表示倒数"><a href="#负数表示倒数" class="header-anchor">#</a> 负数表示倒数</h5> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot;</span>
    1293. 

  1293. <span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Java&quot;</span>
    1294. 

  1294. <span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;p&quot;</span>
    1295. 

  1295. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>如果第一个参数大于第二个参数，<code>slice</code>方法返回一个空字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
    1296. 

  1296. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="_4-5-string-prototype-substring-和slice类似"><a href="#_4-5-string-prototype-substring-和slice类似" class="header-anchor">#</a> 4.5 String.prototype.substring() 和slice类似</h3> <p><code>substring</code>方法<strong>用于从原字符串取出子字符串并返回，不改变原字符串</strong>，跟<code>slice</code>方法很相像。它的第一个参数表示子字符串的<strong>开始位置</strong>，第二个位置表示<strong>结束位置（返回结果不含该位置）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Java&quot;</span>
    1297. 

  1297. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果省略第二个参数，则表示子字符串一直到原字符串的结束。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot; 省略参数二，会取到结束</span>
    1298. 

  1298. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果第一个参数大于第二个参数，<code>substring</code>方法会自动更换两个参数的位置。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot;   // substring自动更换位置，slice则是返回空字符</span>
    1299. 

  1299. <span class="token comment">// 等同于</span>
    1300. 

  1300. <span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot;</span>
    1301. 

  1301. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，调换<code>substring</code>方法的两个参数，都得到同样的结果。</p> <p>如果参数是负数，<code>substring</code>方法会自动将负数转为0。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;JavaScript&quot;  // substring负数自动转为0，slice负数表示倒数</span>
    1302. 

  1302. <span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Java&quot;</span>
    1303. 

  1303. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，第二个例子的参数<code>-3</code>会自动变成<code>0</code>，等同于<code>'JavaScript'.substring(4, 0)</code>。由于第二个参数小于第一个参数，会自动互换位置，所以返回<code>Java</code>。</p> <p>由于这些规则违反直觉，因此<strong>不建议使用<code>substring</code>方法，应该优先使用<code>slice</code></strong>。</p> <h4 id="小总结-substring-和-slice-的区别-有两个。"><a href="#小总结-substring-和-slice-的区别-有两个。" class="header-anchor">#</a> 小总结：substring() 和 slice() 的区别，有两个。</h4> <blockquote><ul><li>负数参数：substring负数自动转为0，slice负数表示倒数</li> <li>参数一大于参数二：substring自动更换位置，slice则是返回空字符</li></ul></blockquote> <h3 id="_4-6-string-prototype-substr-和-slice、substring类似-区别是参数二表示长度"><a href="#_4-6-string-prototype-substr-和-slice、substring类似-区别是参数二表示长度" class="header-anchor">#</a> 4.6 String.prototype.substr() 和 slice、substring类似，区别是参数二表示长度</h3> <p><code>substr</code>方法用于<strong>从原字符串取出子字符串并返回，不改变原字符串</strong>，跟<code>slice</code>和<code>substring</code>方法的作用相同。</p> <p><code>substr</code>方法的第一个参数是子字符串的开始位置（从0开始计算），<strong>第二个参数是子字符串的长度</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substr</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot;</span>
    1304. 

  1304. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果省略第二个参数，则表示子字符串一直到原字符串的结束。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substr</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot; 省略参数二，会取到结束</span>
    1305. 

  1305. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果第一个参数是负数，表示倒数计算的字符位置。如果第二个参数是负数，将被自动转为0，因此会返回空字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substr</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Script&quot; 参数一是负数表示倒数</span>
    1306. 

  1306. <span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">substr</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;  参数二是负数转为0</span>
    1307. 

  1307. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，第二个例子的参数<code>-1</code>自动转为<code>0</code>，表示子字符串长度为<code>0</code>，所以返回空字符串。</p> <h3 id="_4-7-string-prototype-indexof-搜索子串第一个出现的位置-string-prototype-lastindexof-搜索子串最后出现的位置"><a href="#_4-7-string-prototype-indexof-搜索子串第一个出现的位置-string-prototype-lastindexof-搜索子串最后出现的位置" class="header-anchor">#</a> 4.7 String.prototype.indexOf() 搜索子串第一个出现的位置，String.prototype.lastIndexOf() 搜索子串最后出现的位置</h3> <p><code>indexOf</code>方法<strong>用于确定一个字符串在另一个字符串中第一次出现的位置</strong>，<strong>返回结果是匹配开始的位置</strong>。如果返回<code>-1</code>，就表示不匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'hello world'</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">'o'</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    1308. 

  1308. <span class="token string">'JavaScript'</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">'script'</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1309. 

  1309. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>indexOf</code>方法还可以接受第二个参数，表示从该位置开始向后匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'hello world'</span><span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">'o'</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// 7</span>
    1310. 

  1310. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>lastIndexOf</code>方法的用法跟<code>indexOf</code>方法一致，主要的区别是<code>lastIndexOf</code>从尾部开始匹配，<code>indexOf</code>则是从头部开始匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'hello world'</span><span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token string">'o'</span><span class="token punctuation">)</span> <span class="token comment">// 7</span>
    1311. 

  1311. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>另外，<code>lastIndexOf</code>的第二个参数表示从该位置起向前匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'hello world'</span><span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token string">'o'</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    1312. 

  1312. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="_4-8-string-prototype-trim-修剪-两端空格-不改变原字符串"><a href="#_4-8-string-prototype-trim-修剪-两端空格-不改变原字符串" class="header-anchor">#</a> 4.8 String.prototype.trim()  ''修剪'‘两端空格，不改变原字符串</h3> <p><code>trim</code>方法<strong>用于去除字符串两端的空格，返回一个新字符串，不改变原字符串。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'  hello world  '</span><span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1313. 

  1313. <span class="token comment">// &quot;hello world&quot;</span>
    1314. 

  1314. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>该方法去除的不仅是空格，还包括<strong>制表符（<code>\t</code>、<code>\v</code>）、换行符（<code>\n</code>）和回车符（<code>\r</code>）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'\r\nabc \t'</span><span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 'abc'</span>
    1315. 

  1315. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="_4-9-string-prototype-tolowercase-全转小写-string-prototype-touppercase-全转大写-都不改变原字符串"><a href="#_4-9-string-prototype-tolowercase-全转小写-string-prototype-touppercase-全转大写-都不改变原字符串" class="header-anchor">#</a> 4.9 String.prototype.toLowerCase() 全转小写，String.prototype.toUpperCase() 全转大写，都不改变原字符串</h3> <p><code>toLowerCase</code>方法用于将一个字符串<strong>全部转为小写</strong>，<code>toUpperCase</code>则是<strong>全部转为大写</strong>。它们都<strong>返回一个新字符串，不改变原字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'Hello World'</span><span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1316. 

  1316. <span class="token comment">// &quot;hello world&quot;</span>
    1317. 

  1317. 

  1318. <span class="token string">'Hello World'</span><span class="token punctuation">.</span><span class="token function">toUpperCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1319. 

  1319. <span class="token comment">// &quot;HELLO WORLD&quot;</span>
    1320. 

  1320. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="_4-10-string-prototype-match-匹配子串-返回数组-数组带index、input属性"><a href="#_4-10-string-prototype-match-匹配子串-返回数组-数组带index、input属性" class="header-anchor">#</a> 4.10 String.prototype.match() 匹配子串，返回数组（数组带index、input属性）</h3> <p><code>match</code>方法<strong>用于确定原字符串是否匹配某个子字符串，返回一个数组，成员为匹配的第一个字符串</strong>。如果没有找到匹配，则返回<code>null</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'cat, bat, sat, fat'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token string">'at'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;at&quot;]</span>
    1321. 

  1321. <span class="token comment">// [&quot;at&quot;, index: 1, input: &quot;cat, bat, sat, fat&quot;, groups: undefined]</span>
    1322. 

  1322. 

  1323. <span class="token string">'catbatsatbat'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token string">'bat'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;bat&quot;]</span>
    1324. 

  1324. <span class="token comment">//[&quot;bat&quot;, index: 3, input: &quot;catbatsatbat&quot;, groups: undefined]</span>
    1325. 

  1325. <span class="token comment">// index属性为bat首次出现的索引</span>
    1326. 

  1326. 

  1327. <span class="token string">'cat, bat, sat, fat'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token string">'xt'</span><span class="token punctuation">)</span> <span class="token comment">// null</span>
    1328. 

  1328. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>返回的数组还有**<code>index</code>属性<strong>和</strong><code>input</code>属性**，分别表示匹配字符串<strong>开始的位置</strong>和<strong>原始字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> matches <span class="token operator">=</span> <span class="token string">'cat, bat, sat, fat'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token string">'at'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1329. 

  1329. matches<span class="token punctuation">.</span>index <span class="token comment">// 1  子串首次出现的索引位置</span>
    1330. 

  1330. matches<span class="token punctuation">.</span>input <span class="token comment">// &quot;cat, bat, sat, fat&quot;</span>
    1331. 

  1331. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>match</code>方法还可以使用<strong>正则表达式</strong>作为参数，详见《正则表达式》一章。</p> <h3 id="_4-11-string-prototype-search-搜索子串第一次出现的位置-没有则返-1-string-prototype-replace-替换子串-参数二替换参数一-不改变原字符串"><a href="#_4-11-string-prototype-search-搜索子串第一次出现的位置-没有则返-1-string-prototype-replace-替换子串-参数二替换参数一-不改变原字符串" class="header-anchor">#</a> 4.11 String.prototype.search() 搜索子串第一次出现的位置，没有则返-1，String.prototype.replace() 替换子串，参数二替换参数一，不改变原字符串</h3> <p><code>search</code>方法的用法基本等同于<code>match</code>，但是<strong>返回值为匹配的第一个位置。如果没有找到匹配，则返回<code>-1</code>。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'cat, bat, sat, fat'</span><span class="token punctuation">.</span><span class="token function">search</span><span class="token punctuation">(</span><span class="token string">'at'</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1332. 

  1332. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>search</code>方法还可以使用正则表达式作为参数，详见《正则表达式》一节。</p> <p><code>replace</code>方法<strong>用于替换匹配的子字符串</strong>，一般情况下只替换第一个匹配（除非使用带有<code>g</code>修饰符的正则表达式）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'aaa'</span><span class="token punctuation">;</span>
    1333. 

  1333. str<span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;baa&quot; 一般情况只替换第一个，除非带有g的正则</span>
    1334. 

  1334. str <span class="token comment">// 'aaa' 不改变原字符串</span>
    1335. 

  1335. str<span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token string">'c'</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// 'aaa' 匹配不到返回原字符串</span>
    1336. 

  1336. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>replace</code>方法还可以使用正则表达式作为参数，详见《正则表达式》一节。</p> <h3 id="_4-12-string-prototype-split-按给定规则分割字符串-返回数组"><a href="#_4-12-string-prototype-split-按给定规则分割字符串-返回数组" class="header-anchor">#</a> 4.12 String.prototype.split() 按给定规则分割字符串，返回数组</h3> <p><code>split</code>方法<strong>按照给定规则分割字符串，返回一个由分割出来的子字符串组成的数组。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;]</span>
    1337. 

  1337. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果分割规则为空字符串，则返回数组的成员是原字符串的每一个字符。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;|&quot;, &quot;b&quot;, &quot;|&quot;, &quot;c&quot;]</span>
    1338. 

  1338. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果<strong>省略参数，则返回数组的唯一成员就是原字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a|b|c&quot;]</span>
    1339. 

  1339. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果满足分割规则的两个部分紧邻着（即两个分割符中间没有其他字符），则返回数组之中会有一个空字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'a||c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">)</span> <span class="token comment">// ['a', '', 'c']</span>
    1340. 

  1340. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果满足分割规则的部分处于字符串的开头或结尾（即它的前面或后面没有其他字符），则返回数组的第一个或最后一个成员是一个空字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;&quot;, &quot;b&quot;, &quot;c&quot;]</span>
    1341. 

  1341. <span class="token string">'a|b|'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;&quot;]</span>
    1342. 

  1342. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>split</code>方法还可以接受<strong>第二个参数</strong>，<strong>限定返回数组的最大成员数（数组长度）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// []</span>
    1343. 

  1343. <span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;]</span>
    1344. 

  1344. <span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;b&quot;]</span>
    1345. 

  1345. <span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;]</span>
    1346. 

  1346. <span class="token string">'a|b|c'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">'|'</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;b&quot;, &quot;c&quot;]</span>
    1347. 

  1347. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>split</code>方法的第二个参数，决定了返回数组的成员数。</p> <p><code>split</code>方法还可以使用正则表达式作为参数，详见《正则表达式》一节。</p> <h3 id="_4-13-string-prototype-localecompare-比较两个字符串-返回整数。"><a href="#_4-13-string-prototype-localecompare-比较两个字符串-返回整数。" class="header-anchor">#</a> 4.13 String.prototype.localeCompare() 比较两个字符串，返回整数。</h3> <p><code>localeCompare</code>方法 <strong>用于比较两个字符串。它返回一个整数</strong>，<strong>如果小于0，表示第一个字符串小于第二个字符串；如果等于0，表示两者相等；如果大于0，表示第一个字符串大于第二个字符串。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'apple'</span><span class="token punctuation">.</span><span class="token function">localeCompare</span><span class="token punctuation">(</span><span class="token string">'banana'</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1348. 

  1348. <span class="token string">'apple'</span><span class="token punctuation">.</span><span class="token function">localeCompare</span><span class="token punctuation">(</span><span class="token string">'apple'</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1349. 

  1349. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>该方法的最大特点，就是会考虑自然语言的顺序。举例来说，正常情况下，大写的英文字母小于小写字母。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'B'</span> <span class="token operator">&gt;</span> <span class="token string">'a'</span> <span class="token comment">// false</span>
    1350. 

  1350. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，字母<code>B</code>小于字母<code>a</code>。因为 JavaScript 采用的是 Unicode 码点比较，<code>B</code>的码点是66，而<code>a</code>的码点是97。</p> <p>但是，<code>localeCompare</code>方法会考虑自然语言的排序情况，将<code>B</code>排在<code>a</code>的前面。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'B'</span><span class="token punctuation">.</span><span class="token function">localeCompare</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1351. 

  1351. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>localeCompare</code>方法返回整数1，表示<code>B</code>较大。</p> <p><code>localeCompare</code>还可以有第二个参数，指定所使用的语言（默认是英语），然后根据该语言的规则进行比较。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'ä'</span><span class="token punctuation">.</span><span class="token function">localeCompare</span><span class="token punctuation">(</span><span class="token string">'z'</span><span class="token punctuation">,</span> <span class="token string">'de'</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1352. 

  1352. <span class="token string">'ä'</span><span class="token punctuation">.</span><span class="token function">localeCompare</span><span class="token punctuation">(</span><span class="token string">'z'</span><span class="token punctuation">,</span> <span class="token string">'sv'</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1353. 

  1353. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>de</code>表示德语，<code>sv</code>表示瑞典语。德语中，<code>ä</code>小于<code>z</code>，所以返回<code>-1</code>；瑞典语中，<code>ä</code>大于<code>z</code>，所以返回<code>1</code>。</p> <h2 id="八、math对象"><a href="#八、math对象" class="header-anchor">#</a> 八、Math对象</h2> <p><code>Math</code>是 JavaScript 的原生对象，提供各种<strong>数学</strong>功能。该对象<strong>不是构造函数，不能生成实例</strong>，所有的属性和方法都必须在<code>Math</code>对象上调用。</p> <h2 id="_1、静态属性"><a href="#_1、静态属性" class="header-anchor">#</a> 1、静态属性</h2> <p><code>Math</code>对象的静态属性，提供以下一些数学常数。</p> <ul><li><code>Math.E</code>：常数<code>e</code>。</li> <li><code>Math.LN2</code>：2 的自然对数。</li> <li><code>Math.LN10</code>：10 的自然对数。</li> <li><code>Math.LOG2E</code>：以 2 为底的<code>e</code>的对数。</li> <li><code>Math.LOG10E</code>：以 10 为底的<code>e</code>的对数。</li> <li><code>Math.PI</code>：常数<code>π</code>。</li> <li><code>Math.SQRT1_2</code>：0.5 的平方根。</li> <li><code>Math.SQRT2</code>：2 的平方根。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token constant">E</span> <span class="token comment">// 2.718281828459045</span>
    1354. 

  1354. Math<span class="token punctuation">.</span><span class="token constant">LN2</span> <span class="token comment">// 0.6931471805599453</span>
    1355. 

  1355. Math<span class="token punctuation">.</span><span class="token constant">LN10</span> <span class="token comment">// 2.302585092994046</span>
    1356. 

  1356. Math<span class="token punctuation">.</span><span class="token constant">LOG2E</span> <span class="token comment">// 1.4426950408889634</span>
    1357. 

  1357. Math<span class="token punctuation">.</span><span class="token constant">LOG10E</span> <span class="token comment">// 0.4342944819032518</span>
    1358. 

  1358. Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token comment">// 3.141592653589793</span>
    1359. 

  1359. Math<span class="token punctuation">.</span><span class="token constant">SQRT1_2</span> <span class="token comment">// 0.7071067811865476</span>
    1360. 

  1360. Math<span class="token punctuation">.</span><span class="token constant">SQRT2</span> <span class="token comment">// 1.4142135623730951</span>
    1361. 

  1361. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>这些属性都是<strong>只读的</strong>，不能修改。</p> <h2 id="_2、静态方法-3"><a href="#_2、静态方法-3" class="header-anchor">#</a> 2、静态方法</h2> <p><code>Math</code>对象提供以下一些静态方法。</p> <ul><li><code>Math.abs()</code>：绝对值</li> <li><code>Math.ceil()</code>：向上取整</li> <li><code>Math.floor()</code>：向下取整</li> <li><code>Math.max()</code>：最大值</li> <li><code>Math.min()</code>：最小值</li> <li><code>Math.pow()</code>：指数运算</li> <li><code>Math.sqrt()</code>：平方根</li> <li><code>Math.log()</code>：自然对数</li> <li><code>Math.exp()</code>：<code>e</code>的指数</li> <li><code>Math.round()</code>：四舍五入</li> <li><code>Math.random()</code>：随机数</li></ul> <h3 id="_2-1-math-abs-绝对值"><a href="#_2-1-math-abs-绝对值" class="header-anchor">#</a> 2.1 Math.abs() 绝对值</h3> <p><code>Math.abs</code>方法<strong>返回参数值的绝对值</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">abs</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1362. 

  1362. Math<span class="token punctuation">.</span><span class="token function">abs</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1363. 

  1363. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-2-math-max-最大值-math-min-最-小值"><a href="#_2-2-math-max-最大值-math-min-最-小值" class="header-anchor">#</a> 2.2 Math.max() 最大值，Math.min()最 小值</h3> <p><code>Math.max</code>方法<strong>返回参数之中最大的那个值</strong>，<code>Math.min</code><strong>返回最小的那个值</strong>。如果参数为空, <code>Math.min</code>返回<code>Infinity</code>, <code>Math.max</code>返回<code>-Infinity</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// 5</span>
    1364. 

  1364. Math<span class="token punctuation">.</span><span class="token function">min</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1365. 

  1365. Math<span class="token punctuation">.</span><span class="token function">min</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Infinity</span>
    1366. 

  1366. Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// -Infinity</span>
    1367. 

  1367. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="_2-3-math-floor-向下取整-math-ceil-向上取整"><a href="#_2-3-math-floor-向下取整-math-ceil-向上取整" class="header-anchor">#</a> 2.3 Math.floor() 向下取整，Math.ceil() 向上取整</h3> <p><code>Math.floor</code>方法<strong>返回小于参数值的最大整数（地板值）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">floor</span><span class="token punctuation">(</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    1368. 

  1368. Math<span class="token punctuation">.</span><span class="token function">floor</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// -4</span>
    1369. 

  1369. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>Math.ceil</code>方法<strong>返回大于参数值的最小整数（天花板值）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">ceil</span><span class="token punctuation">(</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    1370. 

  1370. Math<span class="token punctuation">.</span><span class="token function">ceil</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// -3</span>
    1371. 

  1371. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这两个方法可以结合起来，实现一个<strong>总是返回数值的整数部分的函数</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1372. 

  1372.   x <span class="token operator">=</span> <span class="token function">Number</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1373. 

  1373.   <span class="token keyword">return</span> x <span class="token operator">&lt;</span> <span class="token number">0</span> <span class="token operator">?</span> Math<span class="token punctuation">.</span><span class="token function">ceil</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span> <span class="token operator">:</span> Math<span class="token punctuation">.</span><span class="token function">floor</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1374. 

  1374. <span class="token punctuation">}</span>
    1375. 

  1375. 

  1376. <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    1377. 

  1377. <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token number">3.5</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    1378. 

  1378. <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token number">3.8</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
    1379. 

  1379. <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// -3</span>
    1380. 

  1380. <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3.5</span><span class="token punctuation">)</span> <span class="token comment">// -3</span>
    1381. 

  1381. <span class="token function">ToInteger</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3.8</span><span class="token punctuation">)</span> <span class="token comment">// -3</span>
    1382. 

  1382. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，不管正数或负数，<code>ToInteger</code>函数总是返回一个数值的整数部分。</p> <h3 id="_2-4-math-round-四舍五入"><a href="#_2-4-math-round-四舍五入" class="header-anchor">#</a> 2.4 Math.round() 四舍五入</h3> <p><code>Math.round</code>方法用于<strong>四舍五入</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token number">0.1</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1383. 

  1383. Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token number">0.5</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1384. 

  1384. Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token number">0.6</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1385. 

  1385. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><strong>注意</strong>，它对负数的处理（主要是对<code>.5</code>的处理）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1.1</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1386. 

  1386. Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1.5</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    1387. 

  1387. Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1.6</span><span class="token punctuation">)</span> <span class="token comment">// -2</span>
    1388. 

  1388. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="_2-5-math-pow-指数运算-次方运算"><a href="#_2-5-math-pow-指数运算-次方运算" class="header-anchor">#</a> 2.5 Math.pow() 指数运算 （次方运算）</h3> <p><code>Math.pow</code>方法返回<strong>以第一个参数为底数、第二个参数为幂的指数值</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 等同于 2 ** 2</span>
    1389. 

  1389. Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 4   2的2次方</span>
    1390. 

  1390. <span class="token comment">// 等同于 2 ** 3</span>
    1391. 

  1391. Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// 8   2的3次方</span>
    1392. 

  1392. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>下面是<strong>计算圆面积的方法</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> radius <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>
    1393. 

  1393. <span class="token keyword">var</span> area <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span>radius<span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1256.6370614359173</span>
    1394. 

  1394. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-6-math-sqrt-平方根"><a href="#_2-6-math-sqrt-平方根" class="header-anchor">#</a> 2.6 Math.sqrt() 平方根</h3> <p><code>Math.sqrt</code>方法<strong>返回参数值的平方根</strong>。如果参数是一个负值，则返回<code>NaN</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">sqrt</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    1395. 

  1395. Math<span class="token punctuation">.</span><span class="token function">sqrt</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    1396. 

  1396. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-7-math-log-自然对数"><a href="#_2-7-math-log-自然对数" class="header-anchor">#</a> 2.7 Math.log() 自然对数</h3> <p><code>Math.log</code>方法返回以<code>e</code>为底的自然对数值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span><span class="token constant">E</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1397. 

  1397. Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span> <span class="token comment">// 2.302585092994046</span>
    1398. 

  1398. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果要计算以10为底的对数，可以先用<code>Math.log</code>求出自然对数，然后除以<code>Math.LN10</code>；求以2为底的对数，可以除以<code>Math.LN2</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token operator">/</span>Math<span class="token punctuation">.</span><span class="token constant">LN10</span> <span class="token comment">// 2</span>
    1399. 

  1399. Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">8</span><span class="token punctuation">)</span><span class="token operator">/</span>Math<span class="token punctuation">.</span><span class="token constant">LN2</span> <span class="token comment">// 3</span>
    1400. 

  1400. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-8-math-exp-e的指数"><a href="#_2-8-math-exp-e的指数" class="header-anchor">#</a> 2.8 Math.exp() <code>e</code>的指数</h3> <p><code>Math.exp</code>方法返回常数<code>e</code>的参数次方。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">exp</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 2.718281828459045</span>
    1401. 

  1401. Math<span class="token punctuation">.</span><span class="token function">exp</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// 20.085536923187668</span>
    1402. 

  1402. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-9-math-random-0到1的随机数-可能等于0-但一定小于1"><a href="#_2-9-math-random-0到1的随机数-可能等于0-但一定小于1" class="header-anchor">#</a> 2.9 Math.random() 0到1的随机数（可能等于0，但一定小于1）</h3> <p><code>Math.random()</code>返回<strong>0到1之间的一个伪随机数</strong>，可能等于0，但是一定小于1。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 0.7151307314634323</span>
    1403. 

  1403. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><strong>任意范围的随机数生成函数</strong>如下。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">getRandomArbitrary</span><span class="token punctuation">(</span><span class="token parameter">min<span class="token punctuation">,</span> max</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1404. 

  1404.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token punctuation">(</span>max <span class="token operator">-</span> min<span class="token punctuation">)</span> <span class="token operator">+</span> min<span class="token punctuation">;</span>
    1405. 

  1405. <span class="token punctuation">}</span>
    1406. 

  1406. 

  1407. <span class="token function">getRandomArbitrary</span><span class="token punctuation">(</span><span class="token number">1.5</span><span class="token punctuation">,</span> <span class="token number">6.5</span><span class="token punctuation">)</span>
    1408. 

  1408. <span class="token comment">// 2.4942810038223864</span>
    1409. 

  1409. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><strong>任意范围的随机整数生成函数</strong>如下。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">getRandomInt</span><span class="token punctuation">(</span><span class="token parameter">min<span class="token punctuation">,</span> max</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1410. 

  1410.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">floor</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token punctuation">(</span>max <span class="token operator">-</span> min <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">+</span> min<span class="token punctuation">;</span>
    1411. 

  1411. <span class="token punctuation">}</span>
    1412. 

  1412. 

  1413. <span class="token function">getRandomInt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// 5</span>
    1414. 

  1414. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>返回<strong>随机字符</strong>的例子如下。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">random_str</span><span class="token punctuation">(</span><span class="token parameter">length</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1415. 

  1415.   <span class="token keyword">var</span> <span class="token constant">ALPHABET</span> <span class="token operator">=</span> <span class="token string">'ABCDEFGHIJKLMNOPQRSTUVWXYZ'</span><span class="token punctuation">;</span>
    1416. 

  1416.   <span class="token constant">ALPHABET</span> <span class="token operator">+=</span> <span class="token string">'abcdefghijklmnopqrstuvwxyz'</span><span class="token punctuation">;</span>
    1417. 

  1417.   <span class="token constant">ALPHABET</span> <span class="token operator">+=</span> <span class="token string">'0123456789-_'</span><span class="token punctuation">;</span>
    1418. 

  1418.   <span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">''</span><span class="token punctuation">;</span>
    1419. 

  1419.   <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> length<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1420. 

  1420.     <span class="token keyword">var</span> rand <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token function">floor</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token constant">ALPHABET</span><span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1421. 

  1421.     str <span class="token operator">+=</span> <span class="token constant">ALPHABET</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span>rand<span class="token punctuation">,</span> rand <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1422. 

  1422.   <span class="token punctuation">}</span>
    1423. 

  1423.   <span class="token keyword">return</span> str<span class="token punctuation">;</span>
    1424. 

  1424. <span class="token punctuation">}</span>
    1425. 

  1425. 

  1426. <span class="token function">random_str</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// &quot;NdQKOr&quot; 参数为指定字符长度</span>
    1427. 

  1427. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，<code>random_str</code>函数接受一个整数作为参数，返回变量<code>ALPHABET</code>内的随机字符所组成的指定长度的字符串。</p> <h3 id="_2-10-三角函数方法"><a href="#_2-10-三角函数方法" class="header-anchor">#</a> 2.10 三角函数方法</h3> <p><code>Math</code>对象还提供一系列三角函数方法。</p> <ul><li><code>Math.sin()</code>：返回参数的正弦（参数为弧度值）</li> <li><code>Math.cos()</code>：返回参数的余弦（参数为弧度值）</li> <li><code>Math.tan()</code>：返回参数的正切（参数为弧度值）</li> <li><code>Math.asin()</code>：返回参数的反正弦（返回值为弧度值）</li> <li><code>Math.acos()</code>：返回参数的反余弦（返回值为弧度值）</li> <li><code>Math.atan()</code>：返回参数的反正切（返回值为弧度值）</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Math<span class="token punctuation">.</span><span class="token function">sin</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1428. 

  1428. Math<span class="token punctuation">.</span><span class="token function">cos</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1429. 

  1429. Math<span class="token punctuation">.</span><span class="token function">tan</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1430. 

  1430. 

  1431. Math<span class="token punctuation">.</span><span class="token function">sin</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">/</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    1432. 

  1432. 

  1433. Math<span class="token punctuation">.</span><span class="token function">asin</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 1.5707963267948966</span>
    1434. 

  1434. Math<span class="token punctuation">.</span><span class="token function">acos</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1435. 

  1435. Math<span class="token punctuation">.</span><span class="token function">atan</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 0.7853981633974483</span>
    1436. 

  1436. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>三角函数的应用相关知识：&lt; https://www.w3cplus.com/javascript/trigonometry-you-must-know.html &gt;</p> <h2 id="九、date对象"><a href="#九、date对象" class="header-anchor">#</a> 九、Date对象</h2> <p><code>Date</code>对象是 JavaScript 原生的时间库。它以国际标准时间（UTC）1970年1月1日00:00:00作为时间的零点，可以表示的时间范围是前后各1亿天（单位为毫秒）。</p> <h2 id="_1、普通函数的用法"><a href="#_1、普通函数的用法" class="header-anchor">#</a> 1、普通函数的用法</h2> <p><code>Date</code>对象可以作为普通函数直接调用，返回一个代表当前时间的<strong>字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1437. 

  1437. <span class="token comment">// &quot;Tue Dec 01 2015 09:34:43 GMT+0800 (CST)&quot; 字符串类型</span>
    1438. 

  1438. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><strong>注意</strong>，即使带有参数，<code>Date</code>作为普通函数使用时，返回的还是当前时间。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">Date</span><span class="token punctuation">(</span><span class="token number">2000</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 作为普通函数使用时带有参数返回的也是当前时间</span>
    1439. 

  1439. <span class="token comment">// &quot;Tue Dec 01 2015 09:34:43 GMT+0800 (CST)&quot;</span>
    1440. 

  1440. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码说明，<strong>无论有没有参数，直接调用<code>Date</code>总是返回当前时间</strong>。</p> <h2 id="_2、构造函数的用法"><a href="#_2、构造函数的用法" class="header-anchor">#</a> 2、构造函数的用法</h2> <p><code>Date</code>还可以当作构造函数使用。对它使用<code>new</code>命令，会返回一个<code>Date</code>对象的实例。如果不加参数，实例代表的就是当前时间。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> today <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1441. 

  1441. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>Date</code>实例有一个独特的地方。其他对象求值的时候，都是默认调用<code>.valueOf()</code>方法，但是<code>Date</code>实例求值的时候，默认调用的是<code>toString()</code>方法。这导致对<code>Date</code>实例求值，返回的是一个字符串，代表该实例对应的时间。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> today <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1442. 

  1442. 

  1443. today
    1444. 

  1444. <span class="token comment">// &quot;Tue Dec 01 2015 09:34:43 GMT+0800 (CST)&quot;</span>
    1445. 

  1445. 

  1446. <span class="token comment">// 等同于</span>
    1447. 

  1447. today<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1448. 

  1448. <span class="token comment">// &quot;Tue Dec 01 2015 09:34:43 GMT+0800 (CST)&quot;</span>
    1449. 

  1449. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>today</code>是<code>Date</code>的实例，<strong>直接求值等同于调用<code>toString</code>方法</strong>。</p> <p>作为构造函数时，<code>Date</code>对象可<strong>以接受多种格式的参数</strong>，返回一个该参数对应的时间实例。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 参数为时间零点开始计算的毫秒数</span>
    1450. 

  1450. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">1378218728000</span><span class="token punctuation">)</span>
    1451. 

  1451. <span class="token comment">// Tue Sep 03 2013 22:32:08 GMT+0800 (CST)</span>
    1452. 

  1452. 

  1453. <span class="token comment">// 参数为日期字符串</span>
    1454. 

  1454. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1455. 

  1455. <span class="token comment">// Sun Jan 06 2013 00:00:00 GMT+0800 (CST)</span>
    1456. 

  1456. 

  1457. <span class="token comment">// 参数为多个整数，</span>
    1458. 

  1458. <span class="token comment">// 代表年、月、日、小时、分钟、秒、毫秒</span>
    1459. 

  1459. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>
    1460. 

  1460. <span class="token comment">// Tue Jan 01 2013 00:00:00 GMT+0800 (CST)</span>
    1461. 

  1461. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>关于<code>Date</code>构造函数的参数，有几点说明。</p> <p><strong>第一点</strong>，参数可以是负整数，代表1970年元旦之前的时间。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1378218728000</span><span class="token punctuation">)</span>
    1462. 

  1462. <span class="token comment">// Fri Apr 30 1926 17:27:52 GMT+0800 (CST)</span>
    1463. 

  1463. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><strong>第二点</strong>，只要是能被<code>Date.parse()</code>方法解析的字符串，都可以当作参数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'2013-2-15'</span><span class="token punctuation">)</span>
    1464. 

  1464. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'2013/2/15'</span><span class="token punctuation">)</span>
    1465. 

  1465. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'02/15/2013'</span><span class="token punctuation">)</span>
    1466. 

  1466. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'2013-FEB-15'</span><span class="token punctuation">)</span>
    1467. 

  1467. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'FEB, 15, 2013'</span><span class="token punctuation">)</span>
    1468. 

  1468. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'FEB 15, 2013'</span><span class="token punctuation">)</span>
    1469. 

  1469. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'February, 15, 2013'</span><span class="token punctuation">)</span>
    1470. 

  1470. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'February 15, 2013'</span><span class="token punctuation">)</span>
    1471. 

  1471. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'15 Feb 2013'</span><span class="token punctuation">)</span>
    1472. 

  1472. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'15, February, 2013'</span><span class="token punctuation">)</span>
    1473. 

  1473. <span class="token comment">// Fri Feb 15 2013 00:00:00 GMT+0800 (CST)</span>
    1474. 

  1474. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面多种日期字符串的写法，返回的都是同一个时间。</p> <p><strong>第三</strong>，参数为年、月、日等多个整数时，年和月是不能省略的，其他参数都可以省略的。也就是说，这时至少需要两个参数，因为如果只使用“年”这一个参数，<code>Date</code>会将其解释为毫秒数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">)</span> <span class="token comment">// 只有一个数字的参数会解释为毫秒数</span>
    1475. 

  1475. <span class="token comment">// Thu Jan 01 1970 08:00:02 GMT+0800 (CST)</span>
    1476. 

  1476. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，2013被解释为毫秒数，而不是年份。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>
    1477. 

  1477. <span class="token comment">// Tue Jan 01 2013 00:00:00 GMT+0800 (CST)</span>
    1478. 

  1478. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span>
    1479. 

  1479. <span class="token comment">// Tue Jan 01 2013 00:00:00 GMT+0800 (CST)</span>
    1480. 

  1480. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>
    1481. 

  1481. <span class="token comment">// Tue Jan 01 2013 00:00:00 GMT+0800 (CST)</span>
    1482. 

  1482. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>
    1483. 

  1483. <span class="token comment">// Tue Jan 01 2013 00:00:00 GMT+0800 (CST)</span>
    1484. 

  1484. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，不管有几个参数，返回的都是2013年1月1日零点。</p> <p>最后，各个参数的取值范围如下。</p> <ul><li>年：使用四位数年份，比如<code>2000</code>。如果写成两位数或个位数，则加上<code>1900</code>，即<code>10</code>代表1910年。如果是负数，表示公元前。</li> <li>月：<strong><code>0</code>表示一月，依次类推，<code>11</code>表示12月</strong>。</li> <li>日：<code>1</code>到<code>31</code>。</li> <li>小时：<code>0</code>到<code>23</code>。</li> <li>分钟：<code>0</code>到<code>59</code>。</li> <li>秒：<code>0</code>到<code>59</code></li> <li>毫秒：<code>0</code>到<code>999</code>。</li></ul> <p>注意，月份从<code>0</code>开始计算，但是，天数从<code>1</code>开始计算。另外，除了日期的默认值为<code>1</code>，小时、分钟、秒钟和毫秒的默认值都是<code>0</code>。</p> <p>这些参数如果超出了正常范围，会被自动折算。比如，如果月设为<code>15</code>，就折算为下一年的4月。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">)</span>
    1485. 

  1485. <span class="token comment">// Tue Apr 01 2014 00:00:00 GMT+0800 (CST)</span>
    1486. 

  1486. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>
    1487. 

  1487. <span class="token comment">// Mon Dec 31 2012 00:00:00 GMT+0800 (CST)</span>
    1488. 

  1488. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码的第二个例子，日期设为<code>0</code>，就代表上个月的最后一天。</p> <p>参数还可以使用负数，表示扣去的时间。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span>
    1489. 

  1489. <span class="token comment">// Sat Dec 01 2012 00:00:00 GMT+0800 (CST)</span>
    1490. 

  1490. <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span>
    1491. 

  1491. <span class="token comment">// Sun Dec 30 2012 00:00:00 GMT+0800 (CST)</span>
    1492. 

  1492. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，分别对月和日使用了负数，表示从基准日扣去相应的时间。</p> <h2 id="_3、日期的运算"><a href="#_3、日期的运算" class="header-anchor">#</a> 3、日期的运算</h2> <p>类型自动转换时，<code>Date</code>实例如果转为数值，则等于对应的毫秒数；如果转为字符串，则等于对应的日期字符串。所以，两个日期实例对象进行减法运算时，返回的是它们间隔的毫秒数；进行加法运算时，返回的是两个字符串连接而成的新字符串。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2000</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1493. 

  1493. <span class="token keyword">var</span> d2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2000</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1494. 

  1494. 

  1495. d2 <span class="token operator">-</span> d1 <span class="token comment">// 减法运算返回毫秒数</span>
    1496. 

  1496. <span class="token comment">// 2678400000</span>
    1497. 

  1497. d2 <span class="token operator">+</span> d1 <span class="token comment">// 加法运算返回日期字符串</span>
    1498. 

  1498. <span class="token comment">// &quot;Sat Apr 01 2000 00:00:00 GMT+0800 (CST)Wed Mar 01 2000 00:00:00 GMT+0800 (CST)&quot;</span>
    1499. 

  1499. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h2 id="_4、静态方法"><a href="#_4、静态方法" class="header-anchor">#</a> 4、静态方法</h2> <h3 id="_4-1-date-now-当前时间戳"><a href="#_4-1-date-now-当前时间戳" class="header-anchor">#</a> 4.1 Date.now() 当前时间戳</h3> <p><code>Date.now</code>方法<strong>返回当前时间距离时间零点（1970年1月1日 00:00:00 UTC）的毫秒数（即时间戳）</strong>，相当于 Unix 时间戳乘以1000。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 1364026285194</span>
    1500. 

  1500. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="_4-2-date-parse-解析日期字符串-返回该时间点的时间戳"><a href="#_4-2-date-parse-解析日期字符串-返回该时间点的时间戳" class="header-anchor">#</a> 4.2 Date.parse() 解析日期字符串，返回该时间点的时间戳</h3> <p><code>Date.parse</code>方法用来<strong>解析日期字符串，返回该时间距离时间零点（1970年1月1日 00:00:00）的毫秒数</strong>。</p> <p>日期字符串应该符合 RFC 2822 和 ISO 8061 这两个标准，即<code>YYYY-MM-DDTHH:mm:ss.sssZ</code>格式，其中最后的<code>Z</code>表示时区。但是，其他格式也可以被解析，请看下面的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'Aug 9, 1995'</span><span class="token punctuation">)</span>
    1501. 

  1501. Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'January 26, 2011 13:51:50'</span><span class="token punctuation">)</span>
    1502. 

  1502. Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'Mon, 25 Dec 1995 13:30:00 GMT'</span><span class="token punctuation">)</span>
    1503. 

  1503. Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'Mon, 25 Dec 1995 13:30:00 +0430'</span><span class="token punctuation">)</span>
    1504. 

  1504. Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'2011-10-10'</span><span class="token punctuation">)</span>
    1505. 

  1505. Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'2011-10-10T14:48:00'</span><span class="token punctuation">)</span>
    1506. 

  1506. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面的日期字符串都可以解析。</p> <p>如果解析失败，返回<code>NaN</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>Date<span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'xxx'</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    1507. 

  1507. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="_4-3-date-utc-以年、月、日等变量作为参数-返回该时间点的时间戳"><a href="#_4-3-date-utc-以年、月、日等变量作为参数-返回该时间点的时间戳" class="header-anchor">#</a> 4.3 Date.UTC() 以年、月、日等变量作为参数，返回该时间点的时间戳</h3> <p><code>Date.UTC</code>方法<strong>接受年、月、日等变量作为参数，返回该时间距离时间零点（1970年1月1日 00:00:00 UTC）的毫秒数。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 格式</span>
    1508. 

  1508. Date<span class="token punctuation">.</span><span class="token constant">UTC</span><span class="token punctuation">(</span>year<span class="token punctuation">,</span> month<span class="token punctuation">[</span><span class="token punctuation">,</span> date<span class="token punctuation">[</span><span class="token punctuation">,</span> hrs<span class="token punctuation">[</span><span class="token punctuation">,</span> min<span class="token punctuation">[</span><span class="token punctuation">,</span> sec<span class="token punctuation">[</span><span class="token punctuation">,</span> ms<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    1509. 

  1509. 

  1510. <span class="token comment">// 用法</span>
    1511. 

  1511. Date<span class="token punctuation">.</span><span class="token constant">UTC</span><span class="token punctuation">(</span><span class="token number">2011</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">567</span><span class="token punctuation">)</span>
    1512. 

  1512. <span class="token comment">// 1293847384567</span>
    1513. 

  1513. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>该方法的参数用法与<code>Date</code>构造函数完全一致，比如月从<code>0</code>开始计算，日期从<code>1</code>开始计算。区别在于<code>Date.UTC</code>方法的参数，会被解释为 UTC 时间（世界标准时间），<code>Date</code>构造函数的参数会被解释为当前时区的时间。</p> <h2 id="_5、实例方法"><a href="#_5、实例方法" class="header-anchor">#</a> 5、实例方法</h2> <p><code>Date</code>的实例对象，有几十个自己的方法，除了<code>valueOf</code>和<code>toString</code>，可以分为以下三类。</p> <ul><li><code>to</code>类：从<code>Date</code>对象返回一个字符串，表示指定的时间。</li> <li><code>get</code>类：获取<code>Date</code>对象的日期和时间。</li> <li><code>set</code>类：设置<code>Date</code>对象的日期和时间。</li></ul> <h3 id="_5-1-date-prototype-valueof-返回时间戳-等同于gettime-。-减法运算默认调用"><a href="#_5-1-date-prototype-valueof-返回时间戳-等同于gettime-。-减法运算默认调用" class="header-anchor">#</a> 5.1 Date.prototype.valueOf() 返回时间戳，等同于getTime()。（减法运算默认调用）</h3> <p><code>valueOf</code>方法<strong>返回实例对象距离时间零点（1970年1月1日00:00:00 UTC）对应的毫秒数</strong>，该方法等同于<code>getTime</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1514. 

  1514. 

  1515. d<span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 1362790014817</span>
    1516. 

  1516. d<span class="token punctuation">.</span><span class="token function">getTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 1362790014817</span>
    1517. 

  1517. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>预期为数值的场合，<code>Date</code>实例会自动调用该方法，所以<strong>可以用下面的方法计算时间的间隔</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> start <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1518. 

  1518. <span class="token comment">// ...</span>
    1519. 

  1519. <span class="token keyword">var</span> end <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1520. 

  1520. <span class="token keyword">var</span> elapsed <span class="token operator">=</span> end <span class="token operator">-</span> start<span class="token punctuation">;</span> <span class="token comment">// 自动调用valueOf()方法，返回时间戳的减法运算</span>
    1521. 

  1521. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="_5-2-to-类方法-返回字符串相关"><a href="#_5-2-to-类方法-返回字符串相关" class="header-anchor">#</a> 5.2 to 类方法（返回字符串相关）</h3> <h4 id="_1-date-prototype-tostring-返回一个完整的日期字符串-直接读取实例默认调用此方法"><a href="#_1-date-prototype-tostring-返回一个完整的日期字符串-直接读取实例默认调用此方法" class="header-anchor">#</a> （1）Date.prototype.toString() 返回一个完整的日期字符串（直接读取实例默认调用此方法）</h4> <p><code>toString</code>方法<strong>返回一个完整的日期字符串</strong>。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>var d = new Date(2013, 0, 1);
    1522. 

  1522. 

  1523. d.toString()
    1524. 

  1524. // &quot;Tue Jan 01 2013 00:00:00 GMT+0800 (CST)&quot;
    1525. 

  1525. d
    1526. 

  1526. // &quot;Tue Jan 01 2013 00:00:00 GMT+0800 (CST)&quot;
    1527. 

  1527. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>因为<code>toString</code>是<strong>默认的调用方法</strong>，所以如果<strong>直接读取<code>Date</code>实例，就相当于调用这个方法</strong>。</p> <h4 id="_2-date-prototype-toutcstring-返回对应的utc时间-世界标准时-比北京时间晚8小时"><a href="#_2-date-prototype-toutcstring-返回对应的utc时间-世界标准时-比北京时间晚8小时" class="header-anchor">#</a> （2）Date.prototype.toUTCString() 返回对应的UTC时间（世界标准时），比北京时间晚8小时</h4> <p><code>toUTCString</code>方法返回对应的 UTC 时间，也就是比北京时间晚8个小时。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1528. 

  1528. 

  1529. d<span class="token punctuation">.</span><span class="token function">toUTCString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1530. 

  1530. <span class="token comment">// &quot;Mon, 31 Dec 2012 16:00:00 GMT&quot;</span>
    1531. 

  1531. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h4 id="_3-date-prototype-toisostring-返回对应时间的-iso8601-写法-utc时间"><a href="#_3-date-prototype-toisostring-返回对应时间的-iso8601-写法-utc时间" class="header-anchor">#</a> （3）Date.prototype.toISOString() 返回对应时间的 ISO8601 写法（UTC时间）</h4> <p><code>toISOString</code>方法返回对应时间的 ISO8601 写法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1532. 

  1532. 

  1533. d<span class="token punctuation">.</span><span class="token function">toISOString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1534. 

  1534. <span class="token comment">// &quot;2012-12-31T16:00:00.000Z&quot;</span>
    1535. 

  1535. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><strong>注意，<code>toISOString</code>方法返回的总是 UTC 时区的时间。</strong></p> <h4 id="_4-date-prototype-tojson-返回符合json格式字符串-与toisostring方法的返回结果完全相同。"><a href="#_4-date-prototype-tojson-返回符合json格式字符串-与toisostring方法的返回结果完全相同。" class="header-anchor">#</a> （4）Date.prototype.toJSON() 返回符合JSON格式字符串，与<code>toISOString</code>方法的返回结果完全相同。</h4> <p><code>toJSON</code>方法返回一个符合 JSON 格式的 ISO 日期字符串，与<code>toISOString</code>方法的返回结果完全相同。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1536. 

  1536. 

  1537. d<span class="token punctuation">.</span><span class="token function">toJSON</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1538. 

  1538. <span class="token comment">// &quot;2012-12-31T16:00:00.000Z&quot;</span>
    1539. 

  1539. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h4 id="_5-date-prototype-todatestring-返回日期字符串-不含时分秒"><a href="#_5-date-prototype-todatestring-返回日期字符串-不含时分秒" class="header-anchor">#</a> （5）Date.prototype.toDateString() 返回日期字符串（不含时分秒）</h4> <p><code>toDateString</code>方法<strong>返回日期字符串（不含小时、分和秒）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1540. 

  1540. d<span class="token punctuation">.</span><span class="token function">toDateString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;Tue Jan 01 2013&quot;</span>
    1541. 

  1541. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h4 id="_6-date-prototype-totimestring-返回时间字符串-不含年月日"><a href="#_6-date-prototype-totimestring-返回时间字符串-不含年月日" class="header-anchor">#</a> （6）Date.prototype.toTimeString() 返回时间字符串（不含年月日）</h4> <p><code>toTimeString</code>方法<strong>返回时间字符串（不含年月日）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1542. 

  1542. d<span class="token punctuation">.</span><span class="token function">toTimeString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;00:00:00 GMT+0800 (CST)&quot;</span>
    1543. 

  1543. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h4 id="_7-本地时间"><a href="#_7-本地时间" class="header-anchor">#</a> （7）本地时间</h4> <p>以下三种方法，可以将 Date 实例转为表示本地时间的字符串。</p> <ul><li><code>Date.prototype.toLocaleString()</code>：完整的本地时间。</li> <li><code>Date.prototype.toLocaleDateString()</code>：本地日期（不含小时、分和秒）。</li> <li><code>Date.prototype.toLocaleTimeString()</code>：本地时间（不含年月日）。</li></ul> <p>下面是用法实例。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1544. 

  1544. 

  1545. d<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1546. 

  1546. <span class="token comment">// 中文版浏览器为&quot;2013年1月1日 上午12:00:00&quot;</span>
    1547. 

  1547. <span class="token comment">// 英文版浏览器为&quot;1/1/2013 12:00:00 AM&quot;</span>
    1548. 

  1548. 

  1549. d<span class="token punctuation">.</span><span class="token function">toLocaleDateString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1550. 

  1550. <span class="token comment">// 中文版浏览器为&quot;2013年1月1日&quot;</span>
    1551. 

  1551. <span class="token comment">// 英文版浏览器为&quot;1/1/2013&quot;</span>
    1552. 

  1552. 

  1553. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    1554. 

  1554. <span class="token comment">// 中文版浏览器为&quot;上午12:00:00&quot;</span>
    1555. 

  1555. <span class="token comment">// 英文版浏览器为&quot;12:00:00 AM&quot;</span>
    1556. 

  1556. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>这三个方法都有两个可选的参数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>dateObj<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token punctuation">[</span>locales<span class="token punctuation">[</span><span class="token punctuation">,</span> options<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    1557. 

  1557. dateObj<span class="token punctuation">.</span><span class="token function">toLocaleDateString</span><span class="token punctuation">(</span><span class="token punctuation">[</span>locales<span class="token punctuation">[</span><span class="token punctuation">,</span> options<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    1558. 

  1558. dateObj<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token punctuation">[</span>locales<span class="token punctuation">[</span><span class="token punctuation">,</span> options<span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    1559. 

  1559. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这两个参数中，<code>locales</code>是一个<strong>指定所用语言的字符串</strong>，<code>options</code>是一个<strong>配置对象</strong>。下面是<code>locales</code>的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1560. 

  1560. 

  1561. d<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1/1/2013, 12:00:00 AM&quot;</span>
    1562. 

  1562. d<span class="token punctuation">.</span><span class="token function">toLocaleString</span><span class="token punctuation">(</span><span class="token string">'zh-CN'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;2013/1/1 上午12:00:00&quot;</span>
    1563. 

  1563. 

  1564. d<span class="token punctuation">.</span><span class="token function">toLocaleDateString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1/1/2013&quot;</span>
    1565. 

  1565. d<span class="token punctuation">.</span><span class="token function">toLocaleDateString</span><span class="token punctuation">(</span><span class="token string">'zh-CN'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;2013/1/1&quot;</span>
    1566. 

  1566. 

  1567. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;12:00:00 AM&quot;</span>
    1568. 

  1568. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token string">'zh-CN'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;上午12:00:00&quot;</span>
    1569. 

  1569. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>下面是<code>options</code>的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token number">2013</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1570. 

  1570. 

  1571. <span class="token comment">// 时间格式</span>
    1572. 

  1572. <span class="token comment">// 下面的设置是，星期和月份为完整文字，年份和日期为数字</span>
    1573. 

  1573. d<span class="token punctuation">.</span><span class="token function">toLocaleDateString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    1574. 

  1574.   <span class="token literal-property property">weekday</span><span class="token operator">:</span> <span class="token string">'long'</span><span class="token punctuation">,</span>
    1575. 

  1575.   <span class="token literal-property property">year</span><span class="token operator">:</span> <span class="token string">'numeric'</span><span class="token punctuation">,</span>
    1576. 

  1576.   <span class="token literal-property property">month</span><span class="token operator">:</span> <span class="token string">'long'</span><span class="token punctuation">,</span>
    1577. 

  1577.   <span class="token literal-property property">day</span><span class="token operator">:</span> <span class="token string">'numeric'</span>
    1578. 

  1578. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1579. 

  1579. <span class="token comment">// &quot;Tuesday, January 1, 2013&quot;</span>
    1580. 

  1580. 

  1581. <span class="token comment">// 指定时区</span>
    1582. 

  1582. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    1583. 

  1583.   <span class="token literal-property property">timeZone</span><span class="token operator">:</span> <span class="token string">'UTC'</span><span class="token punctuation">,</span>
    1584. 

  1584.   <span class="token literal-property property">timeZoneName</span><span class="token operator">:</span> <span class="token string">'short'</span>
    1585. 

  1585. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1586. 

  1586. <span class="token comment">// &quot;4:00:00 PM UTC&quot;</span>
    1587. 

  1587. 

  1588. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    1589. 

  1589.   <span class="token literal-property property">timeZone</span><span class="token operator">:</span> <span class="token string">'Asia/Shanghai'</span><span class="token punctuation">,</span>
    1590. 

  1590.   <span class="token literal-property property">timeZoneName</span><span class="token operator">:</span> <span class="token string">'long'</span>
    1591. 

  1591. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1592. 

  1592. <span class="token comment">// &quot;12:00:00 AM China Standard Time&quot;</span>
    1593. 

  1593. 

  1594. <span class="token comment">// 小时周期为12还是24</span>
    1595. 

  1595. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    1596. 

  1596.   <span class="token literal-property property">hour12</span><span class="token operator">:</span> <span class="token boolean">false</span>
    1597. 

  1597. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1598. 

  1598. <span class="token comment">// &quot;00:00:00&quot;</span>
    1599. 

  1599. 

  1600. d<span class="token punctuation">.</span><span class="token function">toLocaleTimeString</span><span class="token punctuation">(</span><span class="token string">'en-US'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
    1601. 

  1601.   <span class="token literal-property property">hour12</span><span class="token operator">:</span> <span class="token boolean">true</span>
    1602. 

  1602. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1603. 

  1603. <span class="token comment">// &quot;12:00:00 AM&quot;</span>
    1604. 

  1604. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br></div></div><h3 id="_5-3-get类-获取年、月、日、时、分、秒等"><a href="#_5-3-get类-获取年、月、日、时、分、秒等" class="header-anchor">#</a> 5.3 get类 （获取年、月、日、时、分、秒等）</h3> <p><code>Date</code>对象提供了一系列<code>get*</code>方法，用来获取实例对象某个方面的值。</p> <ul><li><code>getTime()</code>：返回实例距离1970年1月1日00:00:00的毫秒数，等同于<code>valueOf</code>方法。<strong>（时间戳）</strong></li> <li><code>getDate()</code>：返回实例对象对应每个月的几号（从1开始）。<strong>（日）</strong></li> <li><code>getDay()</code>：返回星期几，星期日为0，星期一为1，以此类推。<strong>（星期）</strong></li> <li><code>getFullYear()</code>：返回四位的年份。<strong>（年）</strong></li> <li><code>getMonth()</code>：返回月份（0表示1月，11表示12月）。<strong>（月）</strong></li> <li><code>getHours()</code>：返回小时（0-23）。<strong>（时）</strong></li> <li><code>getMilliseconds()</code>：返回毫秒（0-999）。<strong>（毫秒）</strong></li> <li><code>getMinutes()</code>：返回分钟（0-59）。<strong>（分）</strong></li> <li><code>getSeconds()</code>：返回秒（0-59）。<strong>（秒）</strong></li> <li><code>getTimezoneOffset()</code>：返回当前时间与 UTC 的时区差异，以分钟表示，返回结果考虑到了夏令时因素。<strong>（与UTC差异）</strong></li></ul> <p>所有这些<code>get*</code>方法返回的都是<strong>整数</strong>，不同方法返回值的范围不一样。</p> <ul><li>分钟和秒：0 到 59</li> <li>小时：0 到 23</li> <li>星期：0（星期天）到 6（星期六）</li> <li>日期：1 到 31</li> <li>月份：0（一月）到 11（十二月）</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1605. 

  1605. 

  1606. d<span class="token punctuation">.</span><span class="token function">getDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 6</span>
    1607. 

  1607. d<span class="token punctuation">.</span><span class="token function">getMonth</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    1608. 

  1608. d<span class="token punctuation">.</span><span class="token function">getFullYear</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 2013</span>
    1609. 

  1609. d<span class="token punctuation">.</span><span class="token function">getTimezoneOffset</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// -480   UTC时间减去当前时间，单位是分钟</span>
    1610. 

  1610. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，最后一行返回<code>-480</code>，即 UTC 时间减去当前时间，单位是分钟。<code>-480</code>表示 UTC 比当前时间少480分钟，即当前时区比 UTC 早8个小时。</p> <h6 id="例子-计算本年度还剩下多少天"><a href="#例子-计算本年度还剩下多少天" class="header-anchor">#</a> 例子：计算本年度还剩下多少天</h6> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">leftDays</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1611. 

  1611.   <span class="token keyword">var</span> today <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1612. 

  1612.   <span class="token keyword">var</span> endYear <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span>today<span class="token punctuation">.</span><span class="token function">getFullYear</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">31</span><span class="token punctuation">,</span> <span class="token number">23</span><span class="token punctuation">,</span> <span class="token number">59</span><span class="token punctuation">,</span> <span class="token number">59</span><span class="token punctuation">,</span> <span class="token number">999</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1613. 

  1613.   <span class="token keyword">var</span> msPerDay <span class="token operator">=</span> <span class="token number">24</span> <span class="token operator">*</span> <span class="token number">60</span> <span class="token operator">*</span> <span class="token number">60</span> <span class="token operator">*</span> <span class="token number">1000</span><span class="token punctuation">;</span> <span class="token comment">// 一天的毫秒数</span>
    1614. 

  1614.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span><span class="token punctuation">(</span>endYear<span class="token punctuation">.</span><span class="token function">getTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> today<span class="token punctuation">.</span><span class="token function">getTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">/</span> msPerDay<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1615. 

  1615. <span class="token punctuation">}</span>
    1616. 

  1616. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面这些<code>get*</code>方法返回的都是当前时区的时间，<code>Date</code>对象还提供了这些方法对应的 UTC 版本，用来返回 UTC 时间。</p> <p><strong>获取UTC时区相关方法：</strong></p> <ul><li><code>getUTCDate()</code></li> <li><code>getUTCFullYear()</code></li> <li><code>getUTCMonth()</code></li> <li><code>getUTCDay()</code></li> <li><code>getUTCHours()</code></li> <li><code>getUTCMinutes()</code></li> <li><code>getUTCSeconds()</code></li> <li><code>getUTCMilliseconds()</code></li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1617. 

  1617. 

  1618. d<span class="token punctuation">.</span><span class="token function">getDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 6</span>
    1619. 

  1619. d<span class="token punctuation">.</span><span class="token function">getUTCDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 5</span>
    1620. 

  1620. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，实例对象<code>d</code>表示当前时区（东八时区）的1月6日0点0分0秒，这个时间对于当前时区来说是1月6日，所以<code>getDate</code>方法返回6，对于 UTC 时区来说是1月5日，所以<code>getUTCDate</code>方法返回5。</p> <h3 id="_5-4-set-类方法"><a href="#_5-4-set-类方法" class="header-anchor">#</a> 5.4 set 类方法</h3> <p><code>Date</code>对象提供了一系列<code>set*</code>方法，用来设置实例对象的各个方面。</p> <ul><li><code>setDate(date)</code>：设置实例对象对应的每个月的几<strong>号</strong>（1-31），返回改变后毫秒时间戳。</li> <li><code>setFullYear(year [, month, date])</code>：设置四位<strong>年份</strong>。</li> <li><code>setHours(hour [, min, sec, ms])</code>：设置<strong>小时</strong>（0-23）。</li> <li><code>setMilliseconds()</code>：设置<strong>毫秒</strong>（0-999）。</li> <li><code>setMinutes(min [, sec, ms])</code>：设置<strong>分钟</strong>（0-59）。</li> <li><code>setMonth(month [, date])</code>：设置<strong>月份</strong>（0-11）。</li> <li><code>setSeconds(sec [, ms])</code>：设置<strong>秒</strong>（0-59）。</li> <li><code>setTime(milliseconds)</code>：设置<strong>毫秒时间戳</strong>。</li></ul> <p>这些方法基本是跟<code>get*</code>方法一一对应的，但是没有<code>setDay</code>方法，因为星期几是计算出来的，而不是设置的。另外，需要注意的是，凡是涉及到设置月份，都是从0开始算的，即<code>0</code>是1月，<code>11</code>是12月。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span> <span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1621. 

  1621. 

  1622. d <span class="token comment">// Sun Jan 06 2013 00:00:00 GMT+0800 (CST)</span>
    1623. 

  1623. d<span class="token punctuation">.</span><span class="token function">setDate</span><span class="token punctuation">(</span><span class="token number">9</span><span class="token punctuation">)</span> <span class="token comment">// 1357660800000</span>
    1624. 

  1624. d <span class="token comment">// Wed Jan 09 2013 00:00:00 GMT+0800 (CST)</span>
    1625. 

  1625. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><code>set*</code>方法的参数都会自动折算。以<code>setDate</code>为例，如果参数超过当月的最大天数，则向下一个月顺延，如果参数是负数，表示从上个月的最后一天开始减去的天数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1626. 

  1626. 

  1627. d1<span class="token punctuation">.</span><span class="token function">setDate</span><span class="token punctuation">(</span><span class="token number">32</span><span class="token punctuation">)</span> <span class="token comment">// 1359648000000</span>
    1628. 

  1628. d1 <span class="token comment">// Fri Feb 01 2013 00:00:00 GMT+0800 (CST)</span>
    1629. 

  1629. 

  1630. <span class="token keyword">var</span> d2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span> <span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1631. 

  1631. 

  1632. d<span class="token punctuation">.</span><span class="token function">setDate</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 1356796800000</span>
    1633. 

  1633. d <span class="token comment">// Sun Dec 30 2012 00:00:00 GMT+0800 (CST)</span>
    1634. 

  1634. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p><code>**set</code>类方法和<code>get</code>类方法，可以结合使用，得到相对时间。**</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1635. 

  1635. 

  1636. <span class="token comment">// 将日期向后推1000天</span>
    1637. 

  1637. d<span class="token punctuation">.</span><span class="token function">setDate</span><span class="token punctuation">(</span>d<span class="token punctuation">.</span><span class="token function">getDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1638. 

  1638. <span class="token comment">// 将时间设为6小时后</span>
    1639. 

  1639. d<span class="token punctuation">.</span><span class="token function">setHours</span><span class="token punctuation">(</span>d<span class="token punctuation">.</span><span class="token function">getHours</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1640. 

  1640. <span class="token comment">// 将年份设为去年</span>
    1641. 

  1641. d<span class="token punctuation">.</span><span class="token function">setFullYear</span><span class="token punctuation">(</span>d<span class="token punctuation">.</span><span class="token function">getFullYear</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1642. 

  1642. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p><code>set*</code>系列方法除了<code>setTime()</code>，都有对应的 UTC 版本，即<strong>设置 UTC 时区的时间</strong>。</p> <ul><li><code>setUTCDate()</code></li> <li><code>setUTCFullYear()</code></li> <li><code>setUTCHours()</code></li> <li><code>setUTCMilliseconds()</code></li> <li><code>setUTCMinutes()</code></li> <li><code>setUTCMonth()</code></li> <li><code>setUTCSeconds()</code></li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'January 6, 2013'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1643. 

  1643. d<span class="token punctuation">.</span><span class="token function">getUTCHours</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 16</span>
    1644. 

  1644. d<span class="token punctuation">.</span><span class="token function">setUTCHours</span><span class="token punctuation">(</span><span class="token number">22</span><span class="token punctuation">)</span> <span class="token comment">// 1357423200000</span>
    1645. 

  1645. d <span class="token comment">// Sun Jan 06 2013 06:00:00 GMT+0800 (CST)</span>
    1646. 

  1646. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，本地时区（东八时区）的1月6日0点0分，是 UTC 时区的前一天下午16点。设为 UTC 时区的22点以后，就变为本地时区的上午6点。</p> <h2 id="十、regexp-对象"><a href="#十、regexp-对象" class="header-anchor">#</a> 十、RegExp 对象</h2> <p><code>RegExp</code>对象提供正则表达式的功能。</p> <h2 id="_1、概述-6"><a href="#_1、概述-6" class="header-anchor">#</a> 1、概述</h2> <p><strong>正则表达式（regular expression）是一种表达文本模式（即字符串结构）的方法，有点像字符串的模板，常常用来按照“给定模式”匹配文本</strong>。比如，正则表达式给出一个 Email 地址的模式，然后用它来确定一个字符串是否为 Email 地址。JavaScript 的正则表达式体系是参照 Perl 5 建立的。</p> <p><strong>新建正则表达式有两种方法：</strong></p> <p><strong>一种是使用字面量，以斜杠表示开始和结束。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">xyz</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span> <span class="token comment">// 效率较高，且直观，推荐。</span>
    1647. 

  1647. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><strong>另一种是使用<code>RegExp</code>构造函数。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">RegExp</span><span class="token punctuation">(</span><span class="token string">'xyz'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1648. 

  1648. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面两种写法是等价的，都新建了一个内容为<code>xyz</code>的正则表达式对象。它们的主要区别是，第一种方法在引擎编译代码时，就会新建正则表达式，第二种方法在运行时新建正则表达式，所以前者的效率较高。而且，前者比较便利和直观，所以实际应用中，基本上都采用字面量定义正则表达式。</p> <p><code>RegExp</code>构造函数还可以接受第二个参数，表示修饰符（详细解释见下文）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">RegExp</span><span class="token punctuation">(</span><span class="token string">'xyz'</span><span class="token punctuation">,</span> <span class="token string">'i'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// i为修饰符</span>
    1649. 

  1649. <span class="token comment">// 等价于</span>
    1650. 

  1650. <span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">xyz</span><span class="token regex-delimiter">/</span><span class="token regex-flags">i</span></span><span class="token punctuation">;</span>
    1651. 

  1651. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，正则表达式<code>/xyz/</code>有一个修饰符<code>i</code>。</p> <h2 id="_2、实例属性"><a href="#_2、实例属性" class="header-anchor">#</a> 2、实例属性</h2> <p>正则对象的实例属性分成两类。</p> <p>一类是<strong>修饰符相关</strong>，用于了解设置了什么修饰符。</p> <ul><li><code>RegExp.prototype.ignoreCase</code>：返回一个布尔值，表示是否设置了<code>i</code>修饰符。</li> <li><code>RegExp.prototype.global</code>：返回一个布尔值，表示是否设置了<code>g</code>修饰符。</li> <li><code>RegExp.prototype.multiline</code>：返回一个布尔值，表示是否设置了<code>m</code>修饰符。</li> <li><code>RegExp.prototype.flags</code>：返回一个字符串，包含了已经设置的所有修饰符，按字母排序。</li></ul> <p>上面四个属性都是<strong>只读</strong>的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">abc</span><span class="token regex-delimiter">/</span><span class="token regex-flags">igm</span></span><span class="token punctuation">;</span>
    1652. 

  1652. 

  1653. r<span class="token punctuation">.</span>ignoreCase <span class="token comment">// true 是否设置i修饰符</span>
    1654. 

  1654. r<span class="token punctuation">.</span>global <span class="token comment">// true 是否设置g修饰符</span>
    1655. 

  1655. r<span class="token punctuation">.</span>multiline <span class="token comment">// true 是否设置m修饰符</span>
    1656. 

  1656. r<span class="token punctuation">.</span>flags <span class="token comment">// 'gim' 字符串，包含设置的所有修饰符，按字母排序</span>
    1657. 

  1657. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>另一类是与<strong>修饰符无关的属性</strong>，主要是下面两个。</p> <ul><li><code>RegExp.prototype.lastIndex</code>：返回一个整数，表示下一次开始搜索的位置。该属性可读写，但是只在进行连续搜索时有意义，详细介绍请看后文。</li> <li><code>RegExp.prototype.source</code>：返回正则表达式的字符串形式（不包括反斜杠），该属性只读。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">abc</span><span class="token regex-delimiter">/</span><span class="token regex-flags">igm</span></span><span class="token punctuation">;</span>
    1658. 

  1658. 

  1659. r<span class="token punctuation">.</span>lastIndex <span class="token comment">// 0</span>
    1660. 

  1660. r<span class="token punctuation">.</span>source <span class="token comment">// &quot;abc&quot; 正则表达式的字符串形式</span>
    1661. 

  1661. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h2 id="_3、实例方法-3"><a href="#_3、实例方法-3" class="header-anchor">#</a> 3、实例方法</h2> <h3 id="_3-1-regexp-prototype-test-当前模式是否匹配参数字符串-返回布尔值"><a href="#_3-1-regexp-prototype-test-当前模式是否匹配参数字符串-返回布尔值" class="header-anchor">#</a> 3.1 RegExp.prototype.test() 当前模式是否匹配参数字符串，返回布尔值</h3> <p>正则实例对象的<code>test</code>方法<strong>返回一个布尔值，表示当前模式是否能匹配参数字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">cat</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'cats and dogs'</span><span class="token punctuation">)</span> <span class="token comment">// true 验证字符串是否包含cat</span>
    1662. 

  1662. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码验证参数字符串之中是否包含<code>cat</code>，结果返回<code>true</code>。</p> <p>如果正则表达式带有<code>g</code>修饰符，则每一次<code>test</code>方法都从上一次结束的位置开始向后匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">x</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span> <span class="token comment">// 带有g全局匹配修饰符</span>
    1663. 

  1663. <span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'_x_x'</span><span class="token punctuation">;</span>
    1664. 

  1664. 

  1665. r<span class="token punctuation">.</span>lastIndex <span class="token comment">// 0  下一次开始搜索的位置</span>
    1666. 

  1666. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// true 内部原理：从0位置开始匹配，匹配到'x'在1位置，返回true，并把lastIndex属性设置为2，</span>
    1667. 

  1667. 

  1668. r<span class="token punctuation">.</span>lastIndex <span class="token comment">// 2 下一次开始搜索的位置</span>
    1669. 

  1669. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// true  内部原理：从2位置开始匹配，匹配到'x'在3位置，返回true，并把lastIndex属性设置为4</span>
    1670. 

  1670. 

  1671. r<span class="token punctuation">.</span>lastIndex <span class="token comment">// 4 下一次开始搜索的位置</span>
    1672. 

  1672. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// false 内部原理：从4位置开始匹配，匹配不到’x‘,返回false</span>
    1673. 

  1673. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码的正则表达式使用了<code>g</code>修饰符，表示是全局搜索，会有多个结果。接着，三次使用<code>test</code>方法，每一次开始搜索的位置都是上一次匹配的后一个位置。</p> <p>带有<code>g</code>修饰符时，可以通过正则对象的<code>lastIndex</code>属性指定开始搜索的位置。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">x</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1674. 

  1674. <span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'_x_x'</span><span class="token punctuation">;</span>
    1675. 

  1675. 

  1676. r<span class="token punctuation">.</span>lastIndex <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span> <span class="token comment">// lastIndex属性可读写，改写后，test方法将从该位置开始匹配</span>
    1677. 

  1677. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// false</span>
    1678. 

  1678. 

  1679. r<span class="token punctuation">.</span>lastIndex <span class="token comment">// 0  重置为0</span>
    1680. 

  1680. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// true</span>
    1681. 

  1681. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码指定从字符串的第五个位置开始搜索，这个位置为空，所以返回<code>false</code>。同时，<code>lastIndex</code>属性重置为<code>0</code>，所以第二次执行<code>r.test(s)</code>会返回<code>true</code>。</p> <p><strong>注意</strong>，<strong>带有<code>g</code>修饰符时，正则表达式内部会记住上一次的<code>lastIndex</code>属性</strong>，这时不应该更换所要匹配的字符串，否则会有一些难以察觉的错误。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">bb</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1682. 

  1682. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'bb'</span><span class="token punctuation">)</span> <span class="token comment">// true  由于带有g修饰符，匹配后会把lastIndex属性修改为了2</span>
    1683. 

  1683. r<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'-bb-'</span><span class="token punctuation">)</span> <span class="token comment">// false 从2位置开始匹配'bb',并没有匹配到</span>
    1684. 

  1684. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，由于正则表达式<code>r</code>是从上一次的<code>lastIndex</code>位置开始匹配，导致第二次执行<code>test</code>方法时出现预期以外的结果。</p> <p><code>lastIndex</code>属性只对同一个正则表达式有效，所以下面这样写是错误的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> count <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    1685. 

  1685. <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'babaa'</span><span class="token punctuation">)</span><span class="token punctuation">)</span> count<span class="token operator">++</span><span class="token punctuation">;</span>
    1686. 

  1686. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码会导致无限循环，因为<code>while</code>循环的每次匹配条件都是一个新的正则表达式，导致<code>lastIndex</code>属性总是等于0。</p> <p>如果正则模式是一个<strong>空字符串，则匹配所有字符串</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">RegExp</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span>
    1687. 

  1687. <span class="token comment">// true</span>
    1688. 

  1688. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_3-2-regexp-prototype-exec-返回匹配结果-如匹配则返回一个数组-成员是匹配到的子字符串-否则返回null"><a href="#_3-2-regexp-prototype-exec-返回匹配结果-如匹配则返回一个数组-成员是匹配到的子字符串-否则返回null" class="header-anchor">#</a> 3.2 RegExp.prototype.exec() 返回匹配结果，如匹配则返回一个数组，成员是匹配到的子字符串，否则返回null</h3> <p>正则实例对象的<code>exec</code>方法，用来<strong>返回匹配结果。如果发现匹配，就返回一个数组，成员是匹配成功的子字符串，否则返回<code>null</code>。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'_x_x'</span><span class="token punctuation">;</span>
    1689. 

  1689. <span class="token keyword">var</span> r1 <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">x</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1690. 

  1690. <span class="token keyword">var</span> r2 <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">y</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1691. 

  1691. 

  1692. r1<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// [&quot;x&quot;] 返回一个成员</span>
    1693. 

  1693. r2<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// null</span>
    1694. 

  1694. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，正则对象<code>r1</code>匹配成功，返回一个数组，成员是匹配结果；正则对象<code>r2</code>匹配失败，返回<code>null</code>。</p> <p><strong>如果正则表示式包含圆括号（即含有“组匹配”），则返回的数组会包括多个成员</strong>。第一个成员是整个匹配成功的结果，后面的成员就是圆括号对应的匹配成功的组。也就是说，第二个成员对应第一个括号，第三个成员对应第二个括号，以此类推。整个数组的<code>length</code>属性等于组匹配的数量再加1。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'_x_x'</span><span class="token punctuation">;</span>
    1695. 

  1695. <span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">_(x)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span> <span class="token comment">// 含有圆括号，组匹配</span>
    1696. 

  1696. 

  1697. r<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// [&quot;_x&quot;, &quot;x&quot;]  返回多个成员，第一个成员是整个匹配的结果，第二个成员是圆括号匹配的结果</span>
    1698. 

  1698. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码的<code>exec</code>方法，返回一个数组。第一个成员是整个匹配的结果，第二个成员是圆括号匹配的结果。</p> <p><code>exec</code>方法的返回数组还包含以下两个属性：</p> <ul><li><code>input</code>：整个原字符串。</li> <li><code>index</code>：整个模式匹配成功的开始位置（从0开始计数）。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a(b+)a</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1699. 

  1699. <span class="token keyword">var</span> arr <span class="token operator">=</span> r<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span><span class="token string">'_abbba_aba_'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1700. 

  1700. 

  1701. arr <span class="token comment">// [&quot;abbba&quot;, &quot;bbb&quot;] // 第一个成员是整个匹配的结果，第二个成员是圆括号匹配的结果</span>
    1702. 

  1702. 

  1703. arr<span class="token punctuation">.</span>index <span class="token comment">// 1  表示是在1位置开始匹配成功的</span>
    1704. 

  1704. arr<span class="token punctuation">.</span>input <span class="token comment">// &quot;_abbba_aba_&quot;</span>
    1705. 

  1705. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中的<code>index</code>属性等于1，是因为从原字符串的第二个位置开始匹配成功。</p> <p>如果正则表达式加上<code>g</code>修饰符，则可以使用多次<code>exec</code>方法，下一次搜索的位置从上一次匹配成功结束的位置开始。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> reg <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span> <span class="token comment">// 带'g'修饰符</span>
    1706. 

  1706. <span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'abc_abc_abc'</span>
    1707. 

  1707. 

  1708. <span class="token keyword">var</span> r1 <span class="token operator">=</span> reg<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1709. 

  1709. r1 <span class="token comment">// [&quot;a&quot;]</span>
    1710. 

  1710. r1<span class="token punctuation">.</span>index <span class="token comment">// 0 在0位置匹配成功</span>
    1711. 

  1711. reg<span class="token punctuation">.</span>lastIndex <span class="token comment">// 1  下一次匹配开始位置为1</span>
    1712. 

  1712. 

  1713. <span class="token keyword">var</span> r2 <span class="token operator">=</span> reg<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1714. 

  1714. r2 <span class="token comment">// [&quot;a&quot;]</span>
    1715. 

  1715. r2<span class="token punctuation">.</span>index <span class="token comment">// 4 在4位置匹配成功</span>
    1716. 

  1716. reg<span class="token punctuation">.</span>lastIndex <span class="token comment">// 5 下一次匹配开始位置为5</span>
    1717. 

  1717. 

  1718. <span class="token keyword">var</span> r3 <span class="token operator">=</span> reg<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1719. 

  1719. r3 <span class="token comment">// [&quot;a&quot;]</span>
    1720. 

  1720. r3<span class="token punctuation">.</span>index <span class="token comment">// 8 在8位置匹配成功</span>
    1721. 

  1721. reg<span class="token punctuation">.</span>lastIndex <span class="token comment">// 9 下一次匹配开始位置为9</span>
    1722. 

  1722. 

  1723. <span class="token keyword">var</span> r4 <span class="token operator">=</span> reg<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1724. 

  1724. r4 <span class="token comment">// null 没有匹配到返回null</span>
    1725. 

  1725. reg<span class="token punctuation">.</span>lastIndex <span class="token comment">// 0 下一次匹配位置为0</span>
    1726. 

  1726. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>上面代码连续用了四次<code>exec</code>方法，前三次都是从上一次匹配结束的位置向后匹配。当第三次匹配结束以后，整个字符串已经到达尾部，匹配结果返回<code>null</code>，正则实例对象的<code>lastIndex</code>属性也重置为<code>0</code>，意味着第四次匹配将从头开始。</p> <p>利用<code>g</code>修饰符允许多次匹配的特点，可以用一个循环完成全部匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> reg <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1727. 

  1727. <span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'abc_abc_abc'</span>
    1728. 

  1728. 

  1729. <span class="token keyword">while</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1730. 

  1730.   <span class="token keyword">var</span> match <span class="token operator">=</span> reg<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1731. 

  1731.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>match<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    1732. 

  1732.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'#'</span> <span class="token operator">+</span> match<span class="token punctuation">.</span>index <span class="token operator">+</span> <span class="token string">':'</span> <span class="token operator">+</span> match<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1733. 

  1733. <span class="token punctuation">}</span>
    1734. 

  1734. <span class="token comment">// #0:a</span>
    1735. 

  1735. <span class="token comment">// #4:a</span>
    1736. 

  1736. <span class="token comment">// #8:a</span>
    1737. 

  1737. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，只要<code>exec</code>方法不返回<code>null</code>，就会一直循环下去，每次输出匹配的位置和匹配的文本。</p> <p>正则实例对象的<code>lastIndex</code>属性不仅可读，还可写。设置了<code>g</code>修饰符的时候，只要手动设置了<code>lastIndex</code>的值，就会从指定位置开始匹配。</p> <h2 id="_4、字符串的实例方法"><a href="#_4、字符串的实例方法" class="header-anchor">#</a> 4、字符串的实例方法</h2> <p><strong>字符串的实例方法之中，有4种与正则表达式有关。</strong></p> <ul><li><code>String.prototype.match()</code>：返回一个数组，成员是所有匹配的子字符串。</li> <li><code>String.prototype.search()</code>：按照给定的正则表达式进行搜索，返回一个整数，表示匹配开始的位置。</li> <li><code>String.prototype.replace()</code>：按照给定的正则表达式进行替换，返回替换后的字符串。</li> <li><code>String.prototype.split()</code>：按照给定规则进行字符串分割，返回一个数组，包含分割后的各个成员。</li></ul> <h3 id="_4-1-string-prototype-match-匹配-返回匹配结果数组或null"><a href="#_4-1-string-prototype-match-匹配-返回匹配结果数组或null" class="header-anchor">#</a> 4.1 String.prototype.match() 匹配，返回匹配结果数组或null</h3> <p>字符串实例对象的<code>match</code>方法<strong>对字符串进行正则匹配，返回匹配结果</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'_x_x'</span><span class="token punctuation">;</span>
    1738. 

  1738. <span class="token keyword">var</span> r1 <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">x</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1739. 

  1739. <span class="token keyword">var</span> r2 <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">y</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1740. 

  1740. 

  1741. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span>r1<span class="token punctuation">)</span> <span class="token comment">// [&quot;x&quot;] // 未带g修饰符，匹配到一个即返回结果</span>
    1742. 

  1742. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span>r2<span class="token punctuation">)</span> <span class="token comment">// null</span>
    1743. 

  1743. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>从上面代码可以看到，字符串的<code>match</code>方法与正则对象的<code>exec</code>方法非常类似：匹配成功返回一个数组，匹配失败返回<code>null</code>。</p> <p>如果正则表达式带有<code>g</code>修饰符，则该方法与正则对象的<code>exec</code>方法行为不同，会一次性返回所有匹配成功的结果。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'abba'</span><span class="token punctuation">;</span>
    1744. 

  1744. <span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1745. 

  1745. 

  1746. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;, &quot;a&quot;]  带g修饰符，会返回全部结果到数组成员</span>
    1747. 

  1747. r<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;] 正则实例的exec方法只返回一个</span>
    1748. 

  1748. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>设置<strong>正则表达式的<code>lastIndex</code>属性，对<code>match</code>方法无效</strong>，匹配总是从字符串的第一个字符开始。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> r <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a|b</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1749. 

  1749. r<span class="token punctuation">.</span>lastIndex <span class="token operator">=</span> <span class="token number">7</span><span class="token punctuation">;</span>
    1750. 

  1750. <span class="token string">'xaxb'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span> <span class="token comment">// ['a', 'b']</span>
    1751. 

  1751. r<span class="token punctuation">.</span>lastIndex <span class="token comment">// 0</span>
    1752. 

  1752. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码表示，设置正则对象的<code>lastIndex</code>属性是无效的。</p> <h3 id="_4-2-string-prototype-search-返回第一个满足匹配条件的位置-不满足则返-1"><a href="#_4-2-string-prototype-search-返回第一个满足匹配条件的位置-不满足则返-1" class="header-anchor">#</a> 4.2 String.prototype.search() 返回第一个满足匹配条件的位置，不满足则返-1</h3> <p>字符串对象的<code>search</code>方法，<strong>返回第一个满足条件的匹配结果在整个字符串中的位置</strong>。<strong>如果没有任何匹配，则返回<code>-1</code>。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'_x_x'</span><span class="token punctuation">.</span><span class="token function">search</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">x</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span>
    1753. 

  1753. <span class="token comment">// 1</span>
    1754. 

  1754. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，第一个匹配结果出现在字符串的<code>1</code>号位置。</p> <h3 id="_4-3-string-prototype-replace-替换匹配的值-参数一是子串或正则-参数二是替换内容"><a href="#_4-3-string-prototype-replace-替换匹配的值-参数一是子串或正则-参数二是替换内容" class="header-anchor">#</a> 4.3 String.prototype.replace() 替换匹配的值，参数一是子串或正则，参数二是替换内容</h3> <p>字符串对象的<code>replace</code>方法可以<strong>替换匹配的值</strong>。它接受两个参数，第一个是正则表达式，表示搜索模式，第二个是替换的内容。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>str<span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span>search<span class="token punctuation">,</span> replacement<span class="token punctuation">)</span>
    1755. 

  1755. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>正则表达式如果不加<code>g</code>修饰符，就替换第一个匹配成功的值，否则替换所有匹配成功的值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'aaa'</span><span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;baa&quot;</span>
    1756. 

  1756. <span class="token string">'aaa'</span><span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;baa&quot; 不带g修饰符，只匹配并替换第一个</span>
    1757. 

  1757. <span class="token string">'aaa'</span><span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;bbb&quot; 带g修饰符，匹配全局，并替换全部匹配到的值</span>
    1758. 

  1758. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，最后一个正则表达式使用了<code>g</code>修饰符，导致所有的<code>b</code>都被替换掉了。</p> <p><code>replace</code>方法的一个应用，就是消除字符串首尾两端的空格。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'  #id div.class  '</span><span class="token punctuation">;</span>
    1759. 

  1759. 

  1760. str<span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">^\s+|\s+$</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">,</span> <span class="token string">''</span><span class="token punctuation">)</span>
    1761. 

  1761. <span class="token comment">// &quot;#id div.class&quot;</span>
    1762. 

  1762. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>replace</code>方法的<strong>第二个参数可以使用美元符号<code>$</code>，用来指代所替换的内容</strong>。</p> <ul><li><code>$&amp;</code>：匹配的子字符串。</li> <li>$`：匹配结果前面的文本。</li> <li><code>$'</code>：匹配结果后面的文本。</li> <li><code>$n</code>：匹配成功的第<code>n</code>组内容，<code>n</code>是从1开始的自然数。</li> <li><code>$$</code>：指代美元符号<code>$</code>。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'hello world'</span><span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(\w+)\s(\w+)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">,</span> <span class="token string">'$2 $1'</span><span class="token punctuation">)</span>
    1763. 

  1763. <span class="token comment">// &quot;world hello&quot;</span>
    1764. 

  1764. <span class="token comment">/*说明：
    1765. 

  1765. $n 用于组内容，$2为匹配到的第二组，即world； $1为匹配到的第一组，即hello
    1766. 

  1766. */</span>
    1767. 

  1767. 

  1768. <span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'[$`-$&amp;-$\'-$$]'</span><span class="token punctuation">)</span>
    1769. 

  1769. <span class="token comment">// &quot;a[a-b-c-$]c&quot;</span>
    1770. 

  1770. <span class="token comment">/*说明：
    1771. 

  1771. $`为匹配结果 b 前面的文本，即 a
    1772. 

  1772. $&amp;为匹配的子字符串，即 b
    1773. 

  1773. $’为匹配结果 b 后面的文本，即 c
    1774. 

  1774. $$代表$符号本身
    1775. 

  1775. */</span>
    1776. 

  1776. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，第一个例子是将匹配的组互换位置，第二个例子是改写匹配的值。</p> <p><code>replace</code>方法的第<strong>二个参数还可以是一个函数，将每一个匹配内容替换为函数返回值</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'3 and 5'</span><span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">[0-9]+</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">match</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1777. 

  1777.   <span class="token keyword">return</span> <span class="token number">2</span> <span class="token operator">*</span> match<span class="token punctuation">;</span> <span class="token comment">// match参数是匹配到的内容</span>
    1778. 

  1778. <span class="token punctuation">}</span><span class="token punctuation">)</span>
    1779. 

  1779. <span class="token comment">// &quot;6 and 10&quot;</span>
    1780. 

  1780. 

  1781. <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token string">'The quick brown fox jumped over the lazy dog.'</span><span class="token punctuation">;</span>
    1782. 

  1782. <span class="token keyword">var</span> pattern <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">quick|brown|lazy</span><span class="token regex-delimiter">/</span><span class="token regex-flags">ig</span></span><span class="token punctuation">;</span>
    1783. 

  1783. 

  1784. a<span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span>pattern<span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token function">replacer</span><span class="token punctuation">(</span><span class="token parameter">match</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1785. 

  1785.   <span class="token keyword">return</span> match<span class="token punctuation">.</span><span class="token function">toUpperCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1786. 

  1786. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1787. 

  1787. <span class="token comment">// The QUICK BROWN fox jumped over the LAZY dog.</span>
    1788. 

  1788. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>作为<code>replace</code>方法第二个参数的替换函数，可以接受多个参数。其中，第一个参数是捕捉到的内容，第二个参数是捕捉到的组匹配（<strong>有多少个组匹配，就有多少个对应的参数</strong>）。此外，最后还可以添加两个参数，倒数第二个参数是捕捉到的内容在整个字符串中的位置（比如从第五个位置开始），最后一个参数是原字符串。下面是一个网页模板替换的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> prices <span class="token operator">=</span> <span class="token punctuation">{</span>
    1789. 

  1789.   <span class="token string-property property">'p1'</span><span class="token operator">:</span> <span class="token string">'$1.99'</span><span class="token punctuation">,</span>
    1790. 

  1790.   <span class="token string-property property">'p2'</span><span class="token operator">:</span> <span class="token string">'$9.99'</span><span class="token punctuation">,</span>
    1791. 

  1791.   <span class="token string-property property">'p3'</span><span class="token operator">:</span> <span class="token string">'$5.00'</span>
    1792. 

  1792. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    1793. 

  1793. 

  1794. <span class="token keyword">var</span> template <span class="token operator">=</span> <span class="token string">'&lt;span id=&quot;p1&quot;&gt;&lt;/span&gt;'</span>
    1795. 

  1795.   <span class="token operator">+</span> <span class="token string">'&lt;span id=&quot;p2&quot;&gt;&lt;/span&gt;'</span>
    1796. 

  1796.   <span class="token operator">+</span> <span class="token string">'&lt;span id=&quot;p3&quot;&gt;&lt;/span&gt;'</span><span class="token punctuation">;</span>
    1797. 

  1797. 

  1798. template<span class="token punctuation">.</span><span class="token function">replace</span><span class="token punctuation">(</span>
    1799. 

  1799.   <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(&lt;span id=&quot;)(.*?)(&quot;&gt;)(&lt;\/span&gt;)</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">,</span>
    1800. 

  1800.   <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">match<span class="token punctuation">,</span> $1<span class="token punctuation">,</span> $2<span class="token punctuation">,</span> $3<span class="token punctuation">,</span> $4<span class="token punctuation">,</span> index<span class="token punctuation">,</span> str</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    1801. 

  1801.     <span class="token keyword">return</span> $1 <span class="token operator">+</span> $2 <span class="token operator">+</span> $3 <span class="token operator">+</span> prices<span class="token punctuation">[</span>$2<span class="token punctuation">]</span> <span class="token operator">+</span> $4<span class="token punctuation">;</span>
    1802. 

  1802.   <span class="token punctuation">}</span>
    1803. 

  1803. <span class="token punctuation">)</span><span class="token punctuation">;</span>
    1804. 

  1804. <span class="token comment">// &quot;&lt;span id=&quot;p1&quot;&gt;$1.99&lt;/span&gt;&lt;span id=&quot;p2&quot;&gt;$9.99&lt;/span&gt;&lt;span id=&quot;p3&quot;&gt;$5.00&lt;/span&gt;&quot;</span>
    1805. 

  1805. 

  1806. <span class="token comment">// 说明：</span>
    1807. 

  1807. <span class="token comment">// match 整个匹配到的内容：&lt;span id=&quot;p*&quot;&gt;&lt;/span&gt;</span>
    1808. 

  1808. 

  1809. <span class="token comment">// 以下参数分别是匹配到的组内容，有多少组就对应多少个参数</span>
    1810. 

  1810. <span class="token comment">// $1 &lt;span id=&quot;</span>
    1811. 

  1811. <span class="token comment">// $2  p1~p3</span>
    1812. 

  1812. <span class="token comment">// $3  &quot;&gt;</span>
    1813. 

  1813. <span class="token comment">// $4  &lt;/span&gt;</span>
    1814. 

  1814. 

  1815. <span class="token comment">// 后面还可以接受两个参数：</span>
    1816. 

  1816. <span class="token comment">// 倒数第二个是：捕捉到的内容(即match的内容)在整个字符串中的位置</span>
    1817. 

  1817. <span class="token comment">// 最后一个是：原字符串</span>
    1818. 

  1818. 

  1819. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br></div></div><p>上面代码的捕捉模式中，有四个括号，所以会产生四个组匹配，在匹配函数中用<code>$1</code>到<code>$4</code>表示。匹配函数的作用是将价格插入模板中。</p> <h3 id="_4-4-string-prototype-split-按给定规则分割字符串-返回数组"><a href="#_4-4-string-prototype-split-按给定规则分割字符串-返回数组" class="header-anchor">#</a> 4.4 String.prototype.split() 按给定规则分割字符串，返回数组</h3> <p>字符串对象的<code>split</code>方法<strong>按照正则规则分割字符串，返回一个由分割后的各个部分组成的数组。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>str<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span>separator<span class="token punctuation">,</span> <span class="token punctuation">[</span>limit<span class="token punctuation">]</span><span class="token punctuation">)</span>
    1820. 

  1820. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>该方法接受<strong>两个参数</strong>，第一个参数是正则表达式，表示分隔规则，第二个参数是返回数组的最大成员数（数组长度）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 非正则分隔</span>
    1821. 

  1821. <span class="token string">'a,  b,c, d'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">','</span><span class="token punctuation">)</span>
    1822. 

  1822. <span class="token comment">// [ 'a', '  b', 'c', ' d' ]</span>
    1823. 

  1823. 

  1824. <span class="token comment">// 正则分隔，去除多余的空格</span>
    1825. 

  1825. <span class="token string">'a,  b,c, d'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">, *</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span>
    1826. 

  1826. <span class="token comment">// [ 'a', 'b', 'c', 'd' ]</span>
    1827. 

  1827. 

  1828. <span class="token comment">// 指定返回数组的最大成员（数组长度）</span>
    1829. 

  1829. <span class="token string">'a,  b,c, d'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">, *</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span>
    1830. 

  1830. <span class="token punctuation">[</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span> <span class="token punctuation">]</span>
    1831. 

  1831. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码使用正则表达式，去除了子字符串的逗号后面的空格。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 例一</span>
    1832. 

  1832. <span class="token string">'aaa*a*'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a*</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span>
    1833. 

  1833. <span class="token comment">// [ '', '*', '*' ]  分隔符，即被过滤的符号，</span>
    1834. 

  1834. 

  1835. <span class="token comment">// 例二</span>
    1836. 

  1836. <span class="token string">'aaa**a*'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a*</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span>
    1837. 

  1837. <span class="token comment">// [&quot;&quot;, &quot;*&quot;, &quot;*&quot;, &quot;*&quot;]</span>
    1838. 

  1838. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码的分割规则是0次或多次的<code>a</code>，由于正则默认是贪婪匹配，所以例一的第一个分隔符是<code>aaa</code>，第二个分割符是<code>a</code>，将字符串分成三个部分，包含开始处的空字符串。例二的第一个分隔符是<code>aaa</code>，第二个分隔符是0个<code>a</code>（即空字符），第三个分隔符是<code>a</code>，所以将字符串分成四个部分。</p> <p>如果正则表达式<strong>带有括号</strong>，则括号匹配的部分也会作为数组成员返回。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'aaa*a*'</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(a*)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span>
    1839. 

  1839. <span class="token comment">// [ '', 'aaa', '*', 'a', '*' ]</span>
    1840. 

  1840. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码的正则表达式使用了括号，第一个组匹配是<code>aaa</code>，第二个组匹配是<code>a</code>，它们都作为数组成员返回。</p> <h2 id="_5、匹配规则"><a href="#_5、匹配规则" class="header-anchor">#</a> 5、匹配规则</h2> <p>搭配菜鸟教程一起学习更容易理解：<a href="https://www.runoob.com/regexp/regexp-tutorial.html" target="_blank" rel="noopener noreferrer">https://www.runoob.com/regexp/regexp-tutorial.html<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p>正则表达式在线测试与常用正则表达式：<a href="http://c.runoob.com/front-end/854" target="_blank" rel="noopener noreferrer">http://c.runoob.com/front-end/854<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p>正则表达式的规则很复杂，下面一一介绍这些规则。</p> <h3 id="_5-1-字面量字符和元字符"><a href="#_5-1-字面量字符和元字符" class="header-anchor">#</a> 5.1 字面量字符和元字符</h3> <p><strong>大部分字符在正则表达式中，就是字面的含义，比如<code>/a/</code>匹配<code>a</code>，<code>/b/</code>匹配<code>b</code></strong>。如果在正则表达式之中，某个字符只表示它字面的含义（就像前面的<code>a</code>和<code>b</code>），那么它们就叫做“字面量字符”（literal characters）。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>/dog/.test('old dog') // true
    1841. 

  1841. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中正则表达式的<code>dog</code>，就是字面量字符，所以<code>/dog/</code>匹配<code>old dog</code>，因为它就表示<code>d</code>、<code>o</code>、<code>g</code>三个字母连在一起。</p> <p>除了字面量字符以外，还<strong>有一部分字符有特殊含义，不代表字面的意思。它们叫做“元字符”</strong>（metacharacters），主要有以下几个。</p> <h4 id="_1-点字符-类似通配"><a href="#_1-点字符-类似通配" class="header-anchor">#</a> （1）点字符（.)   类似通配</h4> <p>点字符（<code>.</code>）<strong>匹配除回车（<code>\r</code>）、换行(<code>\n</code>) 、行分隔符（<code>\u2028</code>）和段分隔符（<code>\u2029</code>）以外的任何单个字符</strong>。注意，对于码点大于<code>0xFFFF</code>字符，点字符不能正确匹配，会认为这是两个字符。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">c.t</span><span class="token regex-delimiter">/</span></span>        <span class="token comment">//匹配c和t之间包含任意一个字符的情况,注意：一个.只匹配一个字符</span>
    1842. 

  1842. 

  1843. <span class="token string">'cet c2t c-t c.t coot c我t'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">c.t</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">)</span>
    1844. 

  1844. <span class="token comment">// [&quot;cet&quot;, &quot;c2t&quot;, &quot;c-t&quot;, &quot;c.t&quot;, &quot;c我t&quot;]</span>
    1845. 

  1845. 

  1846. <span class="token string">'cet c2t c-t c.t coot c我t'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">c..t</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">)</span>
    1847. 

  1847. <span class="token comment">//[&quot;coot&quot;]</span>
    1848. 

  1848. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>c.t</code>匹配<code>c</code>和<code>t</code>之间包含任意一个字符的情况，只要这三个字符在同一行，比如<code>cat</code>、<code>c2t</code>、<code>c-t</code>等等，但是不匹配<code>coot</code>。</p> <h4 id="_2-位置字符-开始位置和结束位置"><a href="#_2-位置字符-开始位置和结束位置" class="header-anchor">#</a> （2）位置字符(^) ($)  开始位置和结束位置</h4> <p>位置字符用来提示字符所处的位置，主要有两个字符。</p> <ul><li><strong><code>^</code> 表示字符串的开始位置</strong></li> <li><strong><code>$</code> 表示字符串的结束位置</strong></li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// test必须出现在开始位置</span>
    1849. 

  1849. <span class="token operator">/</span><span class="token operator">^</span>test<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'test123'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1850. 

  1850. 

  1851. <span class="token comment">// test必须出现在结束位置</span>
    1852. 

  1852. <span class="token operator">/</span>test$<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'new test'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1853. 

  1853. 

  1854. <span class="token comment">// 从开始位置到结束位置只有test</span>
    1855. 

  1855. <span class="token operator">/</span><span class="token operator">^</span>test$<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'test'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1856. 

  1856. <span class="token operator">/</span><span class="token operator">^</span>test$<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'test test'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1857. 

  1857. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h4 id="_3-选择符-表示-或关系"><a href="#_3-选择符-表示-或关系" class="header-anchor">#</a> （3）选择符（<code>|</code>） 表示''或关系''</h4> <p>竖线符号（<code>|</code>）在正则表达式中表示“<strong>或关系</strong>”（OR），即<code>cat|dog</code>表示匹配<code>cat</code>或<code>dog</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">11|22</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'911'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1858. 

  1858. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，正则表达式指定必须匹配<code>11</code>或<code>22</code>。</p> <p>多个选择符可以联合使用。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 匹配fred、barney、betty之中的一个</span>
    1859. 

  1859. <span class="token operator">/</span>fred<span class="token operator">|</span>barney<span class="token operator">|</span>betty<span class="token operator">/</span>
    1860. 

  1860. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>选择符会包括它前后的多个字符，比如<code>/ab|cd/</code>指的是匹配<code>ab</code>或者<code>cd</code>，而不是指匹配<code>b</code>或者<code>c</code>。如果想修改这个行为，可以使用圆括号。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a( |\t)b</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'a\tb'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1861. 

  1861. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码指的是，<code>a</code>和<code>b</code>之间有一个空格或者一个制表符。</p> <p>其他的元字符还包括<code>\</code>、<code>\*</code>、<code>+</code>、<code>?</code>、<code>()</code>、<code>[]</code>、<code>{}</code>等，将在下文解释。</p> <h3 id="_5-2-转义符-反斜杠"><a href="#_5-2-转义符-反斜杠" class="header-anchor">#</a> 5.2 转义符（\ 反斜杠）</h3> <p>正则表达式中那些有特殊含义的元字符，如果要匹配它们本身，就需要在它们前面要加上反斜杠。比如要匹配<code>+</code>，就要写成<code>\+</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">1+1</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'1+1'</span><span class="token punctuation">)</span>
    1862. 

  1862. <span class="token comment">// false</span>
    1863. 

  1863. 

  1864. <span class="token operator">/</span><span class="token number">1</span>\<span class="token operator">+</span><span class="token number">1</span><span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'1+1'</span><span class="token punctuation">)</span>
    1865. 

  1865. <span class="token comment">// true</span>
    1866. 

  1866. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，第一个正则表达式之所以不匹配，因为加号是元字符，不代表自身。第二个正则表达式使用反斜杠对加号转义，就能匹配成功。</p> <p>正则表达式中，<strong>需要反斜杠转义的，一共有12个字符</strong>：<code>^</code>、<code>.</code>、<code>[</code>、<code>$</code>、<code>(</code>、<code>)</code>、<code>|</code>、<code>*</code>、<code>+</code>、<code>?</code>、<code>{</code>和<code>\</code>。需要特别注意的是，如果使用<code>RegExp</code>方法生成正则对象，转义需要使用两个斜杠，因为字符串内部会先转义一次。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">RegExp</span><span class="token punctuation">(</span><span class="token string">'1\+1'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'1+1'</span><span class="token punctuation">)</span>
    1867. 

  1867. <span class="token comment">// false</span>
    1868. 

  1868. 

  1869. <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">RegExp</span><span class="token punctuation">(</span><span class="token string">'1\\+1'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'1+1'</span><span class="token punctuation">)</span>
    1870. 

  1870. <span class="token comment">// true</span>
    1871. 

  1871. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>RegExp</code>作为构造函数，参数是一个字符串。但是，在字符串内部，反斜杠也是转义字符，所以它会先被反斜杠转义一次，然后再被正则表达式转义一次，因此需要两个反斜杠转义。</p> <h3 id="_5-3-特殊字符"><a href="#_5-3-特殊字符" class="header-anchor">#</a> 5.3 特殊字符</h3> <p>正则表达式对一些不能打印的特殊字符，提供了表达方法。</p> <ul><li><p><code>\cX</code> 表示<code>Ctrl-[X]</code>，其中的<code>X</code>是A-Z之中任一个英文字母，用来匹配控制字符。</p></li> <li><p><code>[\b]</code> 匹配退格键(U+0008)，不要与<code>\b</code>混淆。</p></li> <li><p><code>\n</code> 匹配换行键。</p></li> <li><p><code>\r</code> 匹配回车键。</p></li> <li><p><code>\t</code> 匹配制表符 tab（U+0009）。</p></li> <li><p><code>\v</code> 匹配垂直制表符（U+000B）。</p></li> <li><p><code>\f</code> 匹配换页符（U+000C）。</p></li> <li><p><code>\0</code> 匹配<code>null</code>字符（U+0000）。</p></li> <li><p><code>\xhh</code> 匹配一个以两位十六进制数（<code>\x00</code>-<code>\xFF</code>）表示的字符。</p></li> <li><p><code>\uhhhh</code> 匹配一个以四位十六进制数（<code>\u0000</code>-<code>\uFFFF</code>）表示的 Unicode 字符。</p></li></ul> <h3 id="_5-4-字符类"><a href="#_5-4-字符类" class="header-anchor">#</a> 5.4 字符类</h3> <p>字符类（class）<strong>表示有一系列字符可供选择，只要匹配其中一个就可以了</strong>。所有可供选择的字符都放在<strong>方括号</strong>内，比如<code>[xyz]</code> 表示<code>x</code>、<code>y</code>、<code>z</code>之中任选一个匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">[abc]</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1872. 

  1872. <span class="token operator">/</span><span class="token punctuation">[</span>abc<span class="token punctuation">]</span><span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'apple'</span><span class="token punctuation">)</span> <span class="token comment">// true  a、b、c其中任意一个</span>
    1873. 

  1873. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，字符串<code>hello world</code>不包含<code>a</code>、<code>b</code>、<code>c</code>这三个字母中的任一个，所以返回<code>false</code>；字符串<code>apple</code>包含字母<code>a</code>，所以返回<code>true</code>。</p> <p>有两个字符在字符类中有特殊含义。</p> <h5 id="_1-脱字符-排除"><a href="#_1-脱字符-排除" class="header-anchor">#</a> （1）脱字符（^  排除）</h5> <p>如果方括号内的第一个字符是<code>[^]</code>，则表示<strong>除了字符类之中的字符，其他字符都可以匹配</strong>。比如，<code>[^xyz]</code>表示除了<code>x</code>、<code>y</code>、<code>z</code>之外都可以匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">[^abc]</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'bbc news'</span><span class="token punctuation">)</span> <span class="token comment">// true  字符串中包含了a、b、c以为的字符</span>
    1874. 

  1874. <span class="token operator">/</span><span class="token punctuation">[</span><span class="token operator">^</span>abc<span class="token punctuation">]</span><span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'bbc'</span><span class="token punctuation">)</span> <span class="token comment">// false  字符串中没有除了a、b、c以外的字符</span>
    1875. 

  1875. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，字符串<code>bbc news</code>包含<code>a</code>、<code>b</code>、<code>c</code>以外的其他字符，所以返回<code>true</code>；字符串<code>bbc</code>不包含<code>a</code>、<code>b</code>、<code>c</code>以外的其他字符，所以返回<code>false</code>。</p> <p>如果方括号内没有其他字符，即<strong>只有<code>[^]</code>，就表示匹配一切字符，其中包括换行符</strong>。相比之下，点号作为元字符（<code>.</code>）是不包括换行符的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'Please yes\nmake my day!'</span><span class="token punctuation">;</span>
    1876. 

  1876. 

  1877. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">yes.*day</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// null</span>
    1878. 

  1878. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">yes[^]*day</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [ 'yes\nmake my day']</span>
    1879. 

  1879. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，字符串<code>s</code>含有一个换行符，点号不包括换行符，所以第一个正则表达式匹配失败；第二个正则表达式<code>[^]</code>包含一切字符，所以匹配成功。</p> <blockquote><p>注意，脱字符只有在字符类的第一个位置才有特殊含义，否则就是字面含义。</p></blockquote> <h5 id="_2-连字符-字符连续范围"><a href="#_2-连字符-字符连续范围" class="header-anchor">#</a> （2）连字符（- 字符连续范围）</h5> <p>某些情况下，对于连续序列的字符，连字符（<code>-</code>）用来提供简写形式，表示<strong>字符的连续范围</strong>。比如，<code>[abc]</code>可以写成<code>[a-c]</code>，<code>[0123456789]</code>可以写成<code>[0-9]</code>，同理<code>[A-Z]</code>表示26个大写字母。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a-z</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1880. 

  1880. <span class="token operator">/</span><span class="token punctuation">[</span>a<span class="token operator">-</span>z<span class="token punctuation">]</span><span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'b'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1881. 

  1881. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，当连字号（dash）不出现在方括号之中，就不具备简写的作用，只代表字面的含义，所以不匹配字符<code>b</code>。只有当连字号用在方括号之中，才表示连续的字符序列。</p> <p>以下都是合法的字符类简写形式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token number">0</span><span class="token operator">-</span><span class="token number">9.</span><span class="token punctuation">,</span><span class="token punctuation">]</span>
    1882. 

  1882. <span class="token punctuation">[</span><span class="token number">0</span><span class="token operator">-</span>9a<span class="token operator">-</span>fA<span class="token operator">-</span><span class="token constant">F</span><span class="token punctuation">]</span>
    1883. 

  1883. <span class="token punctuation">[</span>a<span class="token operator">-</span>zA<span class="token operator">-</span><span class="token constant">Z0</span><span class="token operator">-</span><span class="token number">9</span><span class="token operator">-</span><span class="token punctuation">]</span>
    1884. 

  1884. <span class="token punctuation">[</span><span class="token number">1</span><span class="token operator">-</span><span class="token number">31</span><span class="token punctuation">]</span> <span class="token comment">// 注意：不代表1到31，只代表1到3</span>
    1885. 

  1885. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中最后一个字符类<code>[1-31]</code>，不代表<code>1</code>到<code>31</code>，只代表<code>1</code>到<code>3</code>。</p> <p>连字符还可以用来指定 Unicode 字符的范围。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">&quot;\u0130\u0131\u0132&quot;</span><span class="token punctuation">;</span>
    1886. 

  1886. <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">[\u0128-\uFFFF]</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span>
    1887. 

  1887. <span class="token comment">// true</span>
    1888. 

  1888. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>\u0128-\uFFFF</code>表示匹配码点在<code>0128</code>到<code>FFFF</code>之间的所有字符。</p> <p>另外，不要过分使用连字符，设定一个很大的范围，否则很可能选中意料之外的字符。最典型的例子就是<code>[A-z]</code>，表面上它是选中从大写的<code>A</code>到小写的<code>z</code>之间52个字母，但是由于在 ASCII 编码之中，大写字母与小写字母之间还有其他字符，结果就会出现意料之外的结果。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">[A-z]</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'\\'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1889. 

  1889. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，由于反斜杠（'\'）的ASCII码在大写字母与小写字母之间，结果会被选中。</p> <h3 id="_5-5-预定义模式-常见模式简写-d-d-w-w-s-s-b-b"><a href="#_5-5-预定义模式-常见模式简写-d-d-w-w-s-s-b-b" class="header-anchor">#</a> 5.5 预定义模式 (常见模式简写\d\D\w\W\s\S\b\B)</h3> <p>预定义模式指的是<strong>某些常见模式的简写方式</strong>。</p> <ul><li><code>\d</code> 匹配0-9之间的任一数字，相当于<code>[0-9]</code>。<strong>(数字)</strong></li> <li><code>\D</code> 匹配所有0-9以外的字符，相当于<code>[^0-9]</code>。<strong>(非数字)</strong></li> <li><code>\w</code> 匹配任意的字母、数字和下划线，相当于<code>[A-Za-z0-9_]</code>。<strong>(字母、数字、下划线)</strong></li> <li><code>\W</code> 除所有字母、数字和下划线以外的字符，相当于<code>[^A-Za-z0-9_]</code>。<strong>(非：字母、数字、下划线)</strong></li> <li><code>\s</code> 匹配空格（包括换行符、制表符、空格符等），相等于<code>[ \t\r\n\v\f]</code>。<strong>(空格)</strong></li> <li><code>\S</code> 匹配非空格的字符，相当于<code>[^ \t\r\n\v\f]</code>。<strong>(非空格)</strong></li> <li><code>\b</code> 匹配词的边界。<strong>(单词边界)</strong></li> <li><code>\B</code> 匹配非词边界，即在词的内部。<strong>(非单词边界)</strong></li></ul> <p>下面是一些例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// \s 的例子</span>
    1890. 

  1890. <span class="token operator">/</span>\s\w<span class="token operator">*</span><span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span> <span class="token comment">// [&quot; world&quot;]</span>
    1891. 

  1891. 

  1892. <span class="token comment">// \b 的例子</span>
    1893. 

  1893. <span class="token operator">/</span>\bworld<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'hello world'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1894. 

  1894. <span class="token operator">/</span>\bworld<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'hello-world'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1895. 

  1895. <span class="token operator">/</span>\bworld<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'helloworld'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1896. 

  1896. 

  1897. <span class="token comment">// \B 的例子</span>
    1898. 

  1898. <span class="token operator">/</span>\Bworld<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'hello-world'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1899. 

  1899. <span class="token operator">/</span>\Bworld<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'helloworld'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1900. 

  1900. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>\s</code>表示空格，所以匹配结果会包括空格。<code>\b</code>表示词的边界，所以<code>world</code>的词首必须独立（词尾是否独立未指定），才会匹配。同理，<code>\B</code>表示非词的边界，只有<code>world</code>的词首不独立，才会匹配。</p> <p>通常，正则表达式遇到换行符（<code>\n</code>）就会停止匹配。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> html <span class="token operator">=</span> <span class="token string">&quot;&lt;b&gt;Hello&lt;/b&gt;\n&lt;i&gt;world!&lt;/i&gt;&quot;</span><span class="token punctuation">;</span>
    1901. 

  1901. 

  1902. <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">.*</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>html<span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span>
    1903. 

  1903. <span class="token comment">// &quot;&lt;b&gt;Hello&lt;/b&gt;&quot;</span>
    1904. 

  1904. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，字符串<code>html</code>包含一个换行符，结果点字符（<code>.</code>）不匹配换行符，导致匹配结果可能不符合原意。这时使用<code>\s</code>字符类，就能包括换行符。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> html <span class="token operator">=</span> <span class="token string">&quot;&lt;b&gt;Hello&lt;/b&gt;\n&lt;i&gt;world!&lt;/i&gt;&quot;</span><span class="token punctuation">;</span>
    1905. 

  1905. 

  1906. <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">[\S\s]*</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>html<span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span>
    1907. 

  1907. <span class="token comment">// &quot;&lt;b&gt;Hello&lt;/b&gt;\n&lt;i&gt;world!&lt;/i&gt;&quot;</span>
    1908. 

  1908. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>[\S\s]</code>指代一切字符。</p> <h3 id="_5-6-重复类-n-n-n-m"><a href="#_5-6-重复类-n-n-n-m" class="header-anchor">#</a> 5.6 重复类 （ {n} {n,} {n,m} ）</h3> <p>模式的精确匹配次数，使用大括号（<code>{}</code>）表示。<code>{n}</code>表示恰好重复<code>n</code>次，<code>{n,}</code>表示至少重复<code>n</code>次，<code>{n,m}</code>表示重复不少于<code>n</code>次，不多于<code>m</code>次。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">lo{2}k</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'look'</span><span class="token punctuation">)</span> <span class="token comment">// true  2个o</span>
    1909. 

  1909. <span class="token operator">/</span>lo<span class="token punctuation">{</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">}</span>k<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'looook'</span><span class="token punctuation">)</span> <span class="token comment">// true   2到5个o</span>
    1910. 

  1910. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，第一个模式指定<code>o</code>连续出现2次，第二个模式指定<code>o</code>连续出现2次到5次之间。</p> <h3 id="_5-7-量词符"><a href="#_5-7-量词符" class="header-anchor">#</a> 5.7 量词符（?）（*）（+）</h3> <p>量词符用来设定<strong>某个模式出现的次数</strong>。</p> <ul><li><code>?</code> 问号表示某个模式出现<strong>0次或1次</strong>，等同于<code>{0, 1}</code>。</li> <li><code>*</code> 星号表示某个模式出现<strong>0次或多次</strong>，等同于<code>{0,}</code>。</li> <li><code>+</code> 加号表示某个模式出现<strong>1次或多次</strong>，等同于<code>{1,}</code>。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// t 出现0次或1次</span>
    1911. 

  1911. <span class="token operator">/</span>t<span class="token operator">?</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'test'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1912. 

  1912. <span class="token operator">/</span>t<span class="token operator">?</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'est'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1913. 

  1913. 

  1914. <span class="token comment">// t 出现1次或多次</span>
    1915. 

  1915. <span class="token operator">/</span>t<span class="token operator">+</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'test'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1916. 

  1916. <span class="token operator">/</span>t<span class="token operator">+</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'ttest'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1917. 

  1917. <span class="token operator">/</span>t<span class="token operator">+</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'est'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1918. 

  1918. 

  1919. <span class="token comment">// t 出现0次或多次</span>
    1920. 

  1920. <span class="token operator">/</span>t<span class="token operator">*</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'test'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1921. 

  1921. <span class="token operator">/</span>t<span class="token operator">*</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'ttest'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1922. 

  1922. <span class="token operator">/</span>t<span class="token operator">*</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'tttest'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1923. 

  1923. <span class="token operator">/</span>t<span class="token operator">*</span>est<span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'est'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1924. 

  1924. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><h3 id="_5-8-贪婪模式-对量词符可使用非贪婪模式"><a href="#_5-8-贪婪模式-对量词符可使用非贪婪模式" class="header-anchor">#</a> 5.8 贪婪模式（对量词符可使用非贪婪模式）</h3> <p>上一小节的三个量词符，默认情况下都是最大可能匹配，即<strong>匹配直到下一个字符不满足匹配规则为止</strong>。这被称为贪婪模式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'aaa'</span><span class="token punctuation">;</span>
    1925. 

  1925. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a+</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [&quot;aaa&quot;]</span>
    1926. 

  1926. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，模式是<code>/a+/</code>，表示匹配1个<code>a</code>或多个<code>a</code>，那么到底会匹配几个<code>a</code>呢？因为默认是贪婪模式，会一直匹配到字符<code>a</code>不出现为止，所以匹配结果是3个<code>a</code>。</p> <p>如果想将贪婪模式改为非贪婪模式，可以在量词符后面加一个问号。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">'aaa'</span><span class="token punctuation">;</span>
    1927. 

  1927. s<span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">a+?</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [&quot;a&quot;]</span>
    1928. 

  1928. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，模式结尾添加了一个问号<code>/a+?/</code>，这时就改为<strong>非贪婪模式，一旦条件满足，就不再往下匹配</strong>。</p> <p>除了非贪婪模式的加号，还有非贪婪模式的星号（<code>*</code>）和非贪婪模式的问号（<code>?</code>）。</p> <ul><li><code>+?</code>：表示某个模式出现1次或多次，匹配时采用非贪婪模式。</li> <li><code>*?</code>：表示某个模式出现0次或多次，匹配时采用非贪婪模式。</li> <li><code>??</code>：表格某个模式出现0次或1次，匹配时采用非贪婪模式。</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'abb'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">ab*b</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [&quot;abb&quot;]</span>
    1929. 

  1929. <span class="token string">'abb'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">ab*?b</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [&quot;ab&quot;]</span>
    1930. 

  1930. 

  1931. <span class="token string">'abb'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">ab?b</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [&quot;abb&quot;]</span>
    1932. 

  1932. <span class="token string">'abb'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">ab??b</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// [&quot;ab&quot;]</span>
    1933. 

  1933. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="_5-9-修饰符"><a href="#_5-9-修饰符" class="header-anchor">#</a> 5.9 修饰符</h3> <p>修饰符（modifier）表示模式的附加规则，放在正则模式的最尾部。</p> <p>修饰符可以单个使用，也可以多个一起使用。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 单个修饰符</span>
    1934. 

  1934. <span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">test</span><span class="token regex-delimiter">/</span><span class="token regex-flags">i</span></span><span class="token punctuation">;</span>
    1935. 

  1935. 

  1936. <span class="token comment">// 多个修饰符</span>
    1937. 

  1937. <span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">test</span><span class="token regex-delimiter">/</span><span class="token regex-flags">ig</span></span><span class="token punctuation">;</span>
    1938. 

  1938. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h4 id="_1-g-修饰符-全局匹配"><a href="#_1-g-修饰符-全局匹配" class="header-anchor">#</a> （1）g 修饰符 （全局匹配）</h4> <p>默认情况下，第一次匹配成功后，正则对象就停止向下匹配了。<code>g</code>修饰符表示全局匹配（global），加上它以后，正则对象将匹配全部符合条件的结果，主要用于搜索和替换。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">b</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1939. 

  1939. <span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'abba'</span><span class="token punctuation">;</span>
    1940. 

  1940. 

  1941. regex<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    1942. 

  1942. regex<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    1943. 

  1943. regex<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    1944. 

  1944. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，正则模式不含<code>g</code>修饰符，每次都是从字符串头部开始匹配。所以，连续做了三次匹配，都返回<code>true</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> regex <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">b</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1945. 

  1945. <span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'abba'</span><span class="token punctuation">;</span>
    1946. 

  1946. 

  1947. regex<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    1948. 

  1948. regex<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    1949. 

  1949. regex<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    1950. 

  1950. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，正则模式含有<code>g</code>修饰符，每次都是从上一次匹配成功处，开始向后匹配。因为字符串<code>abba</code>只有两个<code>b</code>，所以前两次匹配结果为<code>true</code>，第三次匹配结果为<code>false</code>。</p> <h4 id="_2-i-修饰符-不区分大小写"><a href="#_2-i-修饰符-不区分大小写" class="header-anchor">#</a> （2）i 修饰符 （不区分大小写）</h4> <p>默认情况下，正则对象区分字母的大小写，加上<code>i</code>修饰符以后表示忽略大小写（ignoreCase）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">abc</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'ABC'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1951. 

  1951. <span class="token operator">/</span>abc<span class="token operator">/</span>i<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'ABC'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1952. 

  1952. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示，加了<code>i</code>修饰符以后，不考虑大小写，所以模式<code>abc</code>匹配字符串<code>ABC</code>。</p> <h4 id="_3-m-修饰符-让-和-识别换行符"><a href="#_3-m-修饰符-让-和-识别换行符" class="header-anchor">#</a> （3）m 修饰符（让^和$识别换行符）</h4> <p><code>m</code>修饰符表示多行模式（multiline），会修改<code>^</code>和<code>$</code>的行为。默认情况下（即不加<code>m</code>修饰符时），<code>^</code>和<code>$</code>匹配字符串的开始处和结尾处，加上<code>m</code>修饰符以后，<code>^</code>和<code>$</code>还会匹配行首和行尾，即<code>^</code>和<code>$</code>会识别换行符（<code>\n</code>）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">world$</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'hello world\n'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    1953. 

  1953. <span class="token operator">/</span>world$<span class="token operator">/</span>m<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'hello world\n'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1954. 

  1954. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的代码中，字符串结尾处有一个换行符。如果不加<code>m</code>修饰符，匹配不成功，因为字符串的结尾不是<code>world</code>；加上以后，<code>$</code>可以匹配行尾。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">^b</span><span class="token regex-delimiter">/</span><span class="token regex-flags">m</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'a\nb'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1955. 

  1955. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码要求匹配行首的<code>b</code>，如果不加<code>m</code>修饰符，就相当于<code>b</code>只能处在字符串的开始处。加上<code>m</code>修饰符以后，换行符<code>\n</code>也会被认为是一行的开始。</p> <h3 id="_5-10-组匹配"><a href="#_5-10-组匹配" class="header-anchor">#</a> 5.10 组匹配</h3> <h4 id="_1-概述-圆括号分组"><a href="#_1-概述-圆括号分组" class="header-anchor">#</a> （1）概述 （圆括号分组）</h4> <p>正则表达式的括号表示分组匹配，括号中的模式可以用来匹配分组的内容。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">fred+</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'fredd'</span><span class="token punctuation">)</span> <span class="token comment">// true +加号表示重复字母d</span>
    1956. 

  1956. <span class="token operator">/</span><span class="token punctuation">(</span>fred<span class="token punctuation">)</span><span class="token operator">+</span><span class="token operator">/</span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'fredfred'</span><span class="token punctuation">)</span> <span class="token comment">// true  +加号表示重复单词fred</span>
    1957. 

  1957. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，第一个模式没有括号，结果<code>+</code>只表示重复字母<code>d</code>，第二个模式有括号，结果<code>+</code>就表示匹配<code>fred</code>这个词。</p> <p>下面是另外一个分组捕获的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token string">'abcabc'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(.)b(.)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1958. 

  1958. m
    1959. 

  1959. <span class="token comment">// ['abc', 'a', 'c']</span>
    1960. 

  1960. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，正则表达式<code>/(.)b(.)/</code>一共使用两个括号，第一个括号捕获<code>a</code>，第二个括号捕获<code>c</code>。</p> <p>注意，使用组匹配时，不宜同时使用<code>g</code>修饰符，否则<code>match</code>方法不会捕获分组的内容。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token string">'abcabc'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(.)b(.)</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1961. 

  1961. m <span class="token comment">// ['abc', 'abc']</span>
    1962. 

  1962. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码使用带<code>g</code>修饰符的正则表达式，结果<code>match</code>方法只捕获了匹配整个表达式的部分。这时必须使用正则表达式的<code>exec</code>方法，配合循环，才能读到每一轮匹配的组捕获。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> str <span class="token operator">=</span> <span class="token string">'abcabc'</span><span class="token punctuation">;</span>
    1963. 

  1963. <span class="token keyword">var</span> reg <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(.)b(.)</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1964. 

  1964. <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    1965. 

  1965.   <span class="token keyword">var</span> result <span class="token operator">=</span> reg<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1966. 

  1966.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>result<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    1967. 

  1967.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1968. 

  1968. <span class="token punctuation">}</span>
    1969. 

  1969. <span class="token comment">// [&quot;abc&quot;, &quot;a&quot;, &quot;c&quot;]</span>
    1970. 

  1970. <span class="token comment">// [&quot;abc&quot;, &quot;a&quot;, &quot;c&quot;]</span>
    1971. 

  1971. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>正则表达式内部，还可以用<code>\n</code>引用括号匹配的内容，<code>n</code>是从1开始的自然数，表示对应顺序的括号。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(.)b(.)\1b\2</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">&quot;abcabc&quot;</span><span class="token punctuation">)</span>
    1972. 

  1972. <span class="token comment">// true</span>
    1973. 

  1973. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的代码中，<code>\1</code>表示第一个括号匹配的内容（即<code>a</code>），<code>\2</code>表示第二个括号匹配的内容（即<code>c</code>）。</p> <p>下面是另外一个例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">y(..)(.)\2\1</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'yabccab'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1974. 

  1974. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>括号还可以嵌套。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">y((..)\2)\1</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'yabababab'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    1975. 

  1975. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>\1</code>指向外层括号，<code>\2</code>指向内层括号。</p> <p>组匹配非常有用，下面是一个匹配网页标签的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> tagName <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">&lt;([^&gt;]+)&gt;[^&lt;]*&lt;\/\1&gt;</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1976. 

  1976. 

  1977. tagName<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span><span class="token string">&quot;&lt;b&gt;bold&lt;/b&gt;&quot;</span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span>
    1978. 

  1978. <span class="token comment">// 'b'</span>
    1979. 

  1979. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，圆括号匹配尖括号之中的标签，而<code>\1</code>就表示对应的闭合标签。</p> <p>上面代码略加修改，就能捕获带有属性的标签。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> html <span class="token operator">=</span> <span class="token string">'&lt;b class=&quot;hello&quot;&gt;Hello&lt;/b&gt;&lt;i&gt;world&lt;/i&gt;'</span><span class="token punctuation">;</span>
    1980. 

  1980. <span class="token keyword">var</span> tag <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">&lt;(\w+)([^&gt;]*)&gt;(.*?)&lt;\/\1&gt;</span><span class="token regex-delimiter">/</span><span class="token regex-flags">g</span></span><span class="token punctuation">;</span>
    1981. 

  1981. 

  1982. <span class="token keyword">var</span> match <span class="token operator">=</span> tag<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>html<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1983. 

  1983. 

  1984. match<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span> <span class="token comment">// &quot;b&quot;</span>
    1985. 

  1985. match<span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span> <span class="token comment">// &quot; class=&quot;hello&quot;&quot;</span>
    1986. 

  1986. match<span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span> <span class="token comment">// &quot;Hello&quot;</span>
    1987. 

  1987. 

  1988. match <span class="token operator">=</span> tag<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span>html<span class="token punctuation">)</span><span class="token punctuation">;</span>
    1989. 

  1989. 

  1990. match<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span> <span class="token comment">// &quot;i&quot;</span>
    1991. 

  1991. match<span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span> <span class="token comment">// &quot;&quot;</span>
    1992. 

  1992. match<span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span> <span class="token comment">// &quot;world&quot;</span>
    1993. 

  1993. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><h4 id="_2-非捕获组"><a href="#_2-非捕获组" class="header-anchor">#</a> （2）非捕获组</h4> <p><code>(?:x)</code>称为非捕获组（Non-capturing group），表示<strong>不返回该组匹配的内容，即匹配的结果中不计入这个括号</strong>。</p> <p>非捕获组的作用请考虑这样一个场景，假定需要匹配<code>foo</code>或者<code>foofoo</code>，正则表达式就应该写成<code>/(foo){1, 2}/</code>，但是这样会占用一个组匹配。这时，就可以使用非捕获组，将正则表达式改为<code>/(?:foo){1, 2}/</code>，它的作用与前一个正则是一样的，但是不会单独输出括号内部的内容。</p> <p>请看下面的例子。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(?:.)b(.)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1994. 

  1994. m <span class="token comment">// [&quot;abc&quot;, &quot;c&quot;]</span>
    1995. 

  1995. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中的模式，一共使用了两个括号。其中第一个括号是非捕获组，所以最后返回的结果中没有第一个括号，只有第二个括号匹配的内容。</p> <p>下面是用来分解网址的正则表达式。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 正常匹配</span>
    1996. 

  1996. <span class="token keyword">var</span> url <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(http|ftp):\/\/([^/\r\n]+)(\/[^\r\n]*)?</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    1997. 

  1997. 

  1998. url<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span><span class="token string">'http://google.com/'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    1999. 

  1999. <span class="token comment">// [&quot;http://google.com/&quot;, &quot;http&quot;, &quot;google.com&quot;, &quot;/&quot;]</span>
    2000. 

  2000. 

  2001. <span class="token comment">// 非捕获组匹配</span>
    2002. 

  2002. <span class="token keyword">var</span> url <span class="token operator">=</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">(?:http|ftp):\/\/([^/\r\n]+)(\/[^\r\n]*)?</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">;</span>
    2003. 

  2003. 

  2004. url<span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span><span class="token string">'http://google.com/'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2005. 

  2005. <span class="token comment">// [&quot;http://google.com/&quot;, &quot;google.com&quot;, &quot;/&quot;]</span>
    2006. 

  2006. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面的代码中，前一个正则表达式是正常匹配，第一个括号返回网络协议；后一个正则表达式是非捕获匹配，返回结果中不包括网络协议。</p> <h4 id="_3-先行断言"><a href="#_3-先行断言" class="header-anchor">#</a> （3）先行断言</h4> <p><code>x(?=y)</code>称为先行断言（Positive look-ahead）<strong>，<code>x</code>只有在<code>y</code>前面才匹配，<code>y</code>不会被计入返回结果</strong>。比如，要匹配后面跟着百分号的数字，可以写成<code>/\d+(?=%)/</code>。</p> <p>“先行断言”中，括号里的部分是不会返回的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">b(?=c)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2007. 

  2007. m <span class="token comment">// [&quot;b&quot;]</span>
    2008. 

  2008. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的代码使用了先行断言，<code>b</code>在<code>c</code>前面所以被匹配，但是括号对应的<code>c</code>不会被返回。</p> <h4 id="_4-先行否定断言"><a href="#_4-先行否定断言" class="header-anchor">#</a> （4）先行否定断言</h4> <p><code>x(?!y)</code>称为先行否定断言（Negative look-ahead），<strong><code>x</code>只有不在<code>y</code>前面才匹配，<code>y</code>不会被计入返回结果</strong>。比如，要匹配后面跟的不是百分号的数字，就要写成<code>/\d+(?!%)/</code>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">\d+(?!\.)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">exec</span><span class="token punctuation">(</span><span class="token string">'3.14'</span><span class="token punctuation">)</span>
    2009. 

  2009. <span class="token comment">// [&quot;14&quot;]</span>
    2010. 

  2010. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，正则表达式指定，只有不在小数点前面的数字才会被匹配，因此返回的结果就是<code>14</code>。</p> <p>“先行否定断言”中，括号里的部分是不会返回的。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token string">'abd'</span><span class="token punctuation">.</span><span class="token function">match</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">b(?!c)</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2011. 

  2011. m <span class="token comment">// ['b']</span>
    2012. 

  2012. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的代码使用了先行否定断言，<code>b</code>不在<code>c</code>前面所以被匹配，而且括号对应的<code>d</code>不会被返回。</p> <h2 id="十一、json对象"><a href="#十一、json对象" class="header-anchor">#</a> 十一、JSON对象</h2> <h2 id="_1、json-格式"><a href="#_1、json-格式" class="header-anchor">#</a> 1、JSON 格式</h2> <p>JSON 格式（JavaScript Object Notation 的缩写）是一种用于数据交换的文本格式，2001年由 Douglas Crockford 提出，目的是取代繁琐笨重的 XML 格式。</p> <p>相比 XML 格式，JSON 格式有两个显著的优点：书写简单，一目了然；符合 JavaScript 原生语法，可以由解释引擎直接处理，不用另外添加解析代码。所以，JSON 迅速被接受，已经成为各大网站交换数据的标准格式，并被写入标准。</p> <p><strong>每个 JSON 对象就是一个值，可能是一个数组或对象，也可能是一个原始类型的值</strong>。总之，<strong>只能是一个值</strong>，不能是两个或更多的值。</p> <p>JSON 对值的类型和格式有严格的<strong>规定</strong>。</p> <blockquote><ol><li>复合类型的值只能是数组或对象，不能是函数、正则表达式对象、日期对象。</li> <li>原始类型的值只有四种：字符串、数值（必须以十进制表示）、布尔值和<code>null</code>（不能使用<code>NaN</code>, <code>Infinity</code>, <code>-Infinity</code>和<code>undefined</code>）。</li> <li>字符串必须使用双引号表示，不能使用单引号。</li> <li>对象的键名必须放在双引号里面。</li> <li>数组或对象最后一个成员的后面，不能加逗号。</li></ol></blockquote> <p>以下都是合法的 JSON。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token string">&quot;one&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;two&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;three&quot;</span><span class="token punctuation">]</span>
    2013. 

  2013. 

  2014. <span class="token punctuation">{</span> <span class="token string-property property">&quot;one&quot;</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token string-property property">&quot;two&quot;</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token string-property property">&quot;three&quot;</span><span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span>
    2015. 

  2015. 

  2016. <span class="token punctuation">{</span><span class="token string-property property">&quot;names&quot;</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token string">&quot;张三&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;李四&quot;</span><span class="token punctuation">]</span> <span class="token punctuation">}</span>
    2017. 

  2017. 

  2018. <span class="token punctuation">[</span> <span class="token punctuation">{</span> <span class="token string-property property">&quot;name&quot;</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token string-property property">&quot;name&quot;</span><span class="token operator">:</span> <span class="token string">&quot;李四&quot;</span><span class="token punctuation">}</span> <span class="token punctuation">]</span>
    2019. 

  2019. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>以下都是不合法的 JSON。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span><span class="token punctuation">,</span> <span class="token string-property property">'age'</span><span class="token operator">:</span> <span class="token number">32</span> <span class="token punctuation">}</span>  <span class="token comment">// 属性名必须使用双引号</span>
    2020. 

  2020. 

  2021. <span class="token punctuation">[</span><span class="token number">32</span><span class="token punctuation">,</span> <span class="token number">64</span><span class="token punctuation">,</span> <span class="token number">128</span><span class="token punctuation">,</span> <span class="token number">0xFFF</span><span class="token punctuation">]</span> <span class="token comment">// 不能使用十六进制值</span>
    2022. 

  2022. 

  2023. <span class="token punctuation">{</span> <span class="token string-property property">&quot;name&quot;</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span><span class="token punctuation">,</span> <span class="token string-property property">&quot;age&quot;</span><span class="token operator">:</span> <span class="token keyword">undefined</span> <span class="token punctuation">}</span> <span class="token comment">// 不能使用 undefined</span>
    2024. 

  2024. 

  2025. <span class="token punctuation">{</span> <span class="token string-property property">&quot;name&quot;</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span><span class="token punctuation">,</span>
    2026. 

  2026.   <span class="token string-property property">&quot;birthday&quot;</span><span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'Fri, 26 Aug 2011 07:13:10 GMT'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    2027. 

  2027.   <span class="token string-property property">&quot;getName&quot;</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2028. 

  2028.       <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token punctuation">;</span>
    2029. 

  2029.   <span class="token punctuation">}</span>
    2030. 

  2030. <span class="token punctuation">}</span> <span class="token comment">// 属性值不能使用函数和日期对象</span>
    2031. 

  2031. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p><strong>注意，<code>null</code>、空数组和空对象都是合法的 JSON 值。</strong></p> <h2 id="_2、json-对象"><a href="#_2、json-对象" class="header-anchor">#</a> 2、JSON 对象</h2> <p><code>JSON</code>对象是 JavaScript 的原生对象，用来处理 JSON 格式数据。它有两个静态方法：<code>JSON.stringify()</code>和<code>JSON.parse()</code>。</p> <h2 id="_3、json-stringify-转为json字符串"><a href="#_3、json-stringify-转为json字符串" class="header-anchor">#</a> 3、JSON.stringify() 转为JSON字符串</h2> <h3 id="_3-1-基本用法"><a href="#_3-1-基本用法" class="header-anchor">#</a> 3.1 基本用法</h3> <p><code>JSON.stringify</code>方法用于<strong>将一个值转为 JSON 字符串</strong>。该字符串符合 JSON 格式，并且可以被<code>JSON.parse</code>方法还原。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 单个字符串、数值、布尔值、null、空数组、空对象，都是合法json格式。</span>
    2032. 

  2032. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;abc&quot;&quot; 注意：字符串会转json字符串会有两对双引号</span>
    2033. 

  2033. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// &quot;1&quot;</span>
    2034. 

  2034. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token comment">// &quot;false&quot;</span>
    2035. 

  2035. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// &quot;null&quot;</span>
    2036. 

  2036. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// &quot;[]&quot;</span>
    2037. 

  2037. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// &quot;{}&quot;</span>
    2038. 

  2038. 

  2039. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token string">&quot;false&quot;</span><span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    2040. 

  2040. <span class="token comment">// '[1,&quot;false&quot;,false]'</span>
    2041. 

  2041. 

  2042. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token string">&quot;张三&quot;</span> <span class="token punctuation">}</span><span class="token punctuation">)</span>
    2043. 

  2043. <span class="token comment">// '{&quot;name&quot;:&quot;张三&quot;}'</span>
    2044. 

  2044. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码将各种类型的值，转成 JSON 字符串。</p> <p>注意，<strong>对于原始类型的字符串，转换结果会带双引号</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">&quot;foo&quot;</span> <span class="token comment">// false</span>
    2045. 

  2045. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">&quot;\&quot;foo\&quot;&quot;</span> <span class="token comment">// true</span>
    2046. 

  2046. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，字符串<code>foo</code>，被转成了<code>&quot;\&quot;foo\&quot;&quot;</code>。这是因为将来还原的时候，内层双引号可以让 JavaScript 引擎知道，这是一个字符串，而不是其他类型的值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token comment">// &quot;false&quot;</span>
    2047. 

  2047. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token string">'false'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;\&quot;false\&quot;&quot;</span>
    2048. 

  2048. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，如果不是内层的双引号，将来还原的时候，引擎就无法知道原始值是布尔值还是字符串。</p> <p><strong>如果对象的属性是<code>undefined</code>、函数或 XML 对象，该属性会被<code>JSON.stringify</code>过滤</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    2049. 

  2049.   <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span>
    2050. 

  2050.   <span class="token function-variable function">b</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    2051. 

  2051. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    2052. 

  2052. 

  2053. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// &quot;{}&quot; 对象属性是undefined、函数或 XML 对象会被过滤掉</span>
    2054. 

  2054. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，对象<code>obj</code>的<code>a</code>属性是<code>undefined</code>，而<code>b</code>属性是一个函数，结果都被<code>JSON.stringify</code>过滤。</p> <p><strong>如果数组的成员是<code>undefined</code>、函数或 XML 对象，则这些值被转成<code>null</code>。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    2055. 

  2055. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span> <span class="token comment">// &quot;[null,null]&quot; 数组成员是undefined、函数或 XML 对象会被转成null</span>
    2056. 

  2056. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，数组<code>arr</code>的成员是<code>undefined</code>和函数，它们都被转成了<code>null</code>。</p> <p><strong>正则对象会被转成空对象</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">foo</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// &quot;{}&quot;</span>
    2057. 

  2057. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>JSON.stringify</code>方法会<strong>忽略对象的不可遍历的属性</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    2058. 

  2058. Object<span class="token punctuation">.</span><span class="token function">defineProperties</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
    2059. 

  2059.   <span class="token string-property property">'foo'</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    2060. 

  2060.     <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    2061. 

  2061.     <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">true</span>
    2062. 

  2062.   <span class="token punctuation">}</span><span class="token punctuation">,</span>
    2063. 

  2063.   <span class="token string-property property">'bar'</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    2064. 

  2064.     <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
    2065. 

  2065.     <span class="token literal-property property">enumerable</span><span class="token operator">:</span> <span class="token boolean">false</span>
    2066. 

  2066.   <span class="token punctuation">}</span>
    2067. 

  2067. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2068. 

  2068. 

  2069. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;{&quot;foo&quot;:1}&quot;</span>
    2070. 

  2070. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，<code>bar</code>是<code>obj</code>对象的不可遍历属性，<code>JSON.stringify</code>方法会忽略这个属性。</p> <h3 id="_3-2-第二个参数-一个数组-对象属性白名单-或一个函数-改写返回值"><a href="#_3-2-第二个参数-一个数组-对象属性白名单-或一个函数-改写返回值" class="header-anchor">#</a> 3.2 第二个参数 （一个数组，对象属性白名单；或一个函数，改写返回值）</h3> <p><code>JSON.stringify</code>方法还<strong>可以接受一个数组，作为第二个参数，指定需要转成字符串的属性</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    2071. 

  2071.   <span class="token string-property property">'prop1'</span><span class="token operator">:</span> <span class="token string">'value1'</span><span class="token punctuation">,</span>
    2072. 

  2072.   <span class="token string-property property">'prop2'</span><span class="token operator">:</span> <span class="token string">'value2'</span><span class="token punctuation">,</span>
    2073. 

  2073.   <span class="token string-property property">'prop3'</span><span class="token operator">:</span> <span class="token string">'value3'</span>
    2074. 

  2074. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    2075. 

  2075. 

  2076. <span class="token keyword">var</span> selectedProperties <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'prop1'</span><span class="token punctuation">,</span> <span class="token string">'prop2'</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// 指定prop1、prop2属性转成字符串</span>
    2077. 

  2077. 

  2078. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> selectedProperties<span class="token punctuation">)</span>
    2079. 

  2079. <span class="token comment">// &quot;{&quot;prop1&quot;:&quot;value1&quot;,&quot;prop2&quot;:&quot;value2&quot;}&quot;</span>
    2080. 

  2080. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>JSON.stringify</code>方法的第二个参数指定，只转<code>prop1</code>和<code>prop2</code>两个属性。</p> <p>这个<strong>类似白名单的数组，只对对象的属性有效，对数组无效。</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token string">'0'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    2081. 

  2081. <span class="token comment">// &quot;[&quot;a&quot;,&quot;b&quot;]&quot;</span>
    2082. 

  2082. 

  2083. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token number">0</span><span class="token operator">:</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token operator">:</span> <span class="token string">'b'</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token string">'0'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    2084. 

  2084. <span class="token comment">// &quot;{&quot;0&quot;:&quot;a&quot;}&quot;</span>
    2085. 

  2085. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，第二个参数指定 JSON 格式只转<code>0</code>号属性，实际上对数组是无效的，只对对象有效。</p> <p>第二个参数还<strong>可以是一个函数</strong>，用来更改<code>JSON.stringify</code>的返回值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 接收两个参数：键名、键值</span>
    2086. 

  2086.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> value <span class="token operator">===</span> <span class="token string">&quot;number&quot;</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2087. 

  2087.     value <span class="token operator">=</span> <span class="token number">2</span> <span class="token operator">*</span> value<span class="token punctuation">;</span>
    2088. 

  2088.   <span class="token punctuation">}</span>
    2089. 

  2089.   <span class="token keyword">return</span> value<span class="token punctuation">;</span>
    2090. 

  2090. <span class="token punctuation">}</span>
    2091. 

  2091. 

  2092. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> f<span class="token punctuation">)</span>
    2093. 

  2093. <span class="token comment">// '{&quot;a&quot;: 2,&quot;b&quot;: 4}'</span>
    2094. 

  2094. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中的<code>f</code>函数，<strong>接受两个参数</strong>，分别是被转换的对象的<strong>键名和键值</strong>。如果键值是数值，就将它乘以<code>2</code>，否则就原样返回。</p> <p>注意，这个处理函数是<strong>递归处理所有的键</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    2095. 

  2095. 

  2096. <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 每一次处理的对象，都是前一次返回的值</span>
    2097. 

  2097.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">&quot;[&quot;</span><span class="token operator">+</span> key <span class="token operator">+</span><span class="token string">&quot;]:&quot;</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    2098. 

  2098.   <span class="token keyword">return</span> value<span class="token punctuation">;</span>
    2099. 

  2099. <span class="token punctuation">}</span>
    2100. 

  2100. 

  2101. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>o<span class="token punctuation">,</span> f<span class="token punctuation">)</span>
    2102. 

  2102. <span class="token comment">// []:[object Object]  第一次键名 空，键值是o整个对象</span>
    2103. 

  2103. <span class="token comment">// [a]:[object Object] 第二次键名 a，键值是{b:1}</span>
    2104. 

  2104. <span class="token comment">// [b]:1               第三次键名 b，键值是 1</span>
    2105. 

  2105. <span class="token comment">// '{&quot;a&quot;:{&quot;b&quot;:1}}'</span>
    2106. 

  2106. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，对象<code>o</code>一共会被<code>f</code>函数处理三次，最后那行是<code>JSON.stringify</code>的输出。第一次键名为空，键值是整个对象<code>o</code>；第二次键名为<code>a</code>，键值是<code>{b: 1}</code>；第三次键名为<code>b</code>，键值为1。</p> <p>递归处理中，<strong>每一次处理的对象，都是前一次返回的值</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    2107. 

  2107. 

  2108. <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2109. 

  2109.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> value <span class="token operator">===</span> <span class="token string">'object'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2110. 

  2110.     <span class="token keyword">return</span> <span class="token punctuation">{</span><span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    2111. 

  2111.   <span class="token punctuation">}</span>
    2112. 

  2112.   <span class="token keyword">return</span> value <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">;</span>
    2113. 

  2113. <span class="token punctuation">}</span>
    2114. 

  2114. 

  2115. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>o<span class="token punctuation">,</span> f<span class="token punctuation">)</span>
    2116. 

  2116. <span class="token comment">// &quot;{&quot;b&quot;: 4}&quot;</span>
    2117. 

  2117. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>f</code>函数修改了对象<code>o</code>，接着<code>JSON.stringify</code>方法就递归处理修改后的对象<code>o</code>。</p> <p>如果<strong>处理函数返回<code>undefined</code>或没有返回值，则该属性会被忽略</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2118. 

  2118.   <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">&quot;string&quot;</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2119. 

  2119.     <span class="token keyword">return</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span>
    2120. 

  2120.   <span class="token punctuation">}</span>
    2121. 

  2121.   <span class="token keyword">return</span> value<span class="token punctuation">;</span>
    2122. 

  2122. <span class="token punctuation">}</span>
    2123. 

  2123. 

  2124. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token string">&quot;abc&quot;</span><span class="token punctuation">,</span> <span class="token literal-property property">b</span><span class="token operator">:</span> <span class="token number">123</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> f<span class="token punctuation">)</span>
    2125. 

  2125. <span class="token comment">// '{&quot;b&quot;: 123}'</span>
    2126. 

  2126. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>a</code>属性经过处理后，返回<code>undefined</code>，于是该属性被忽略了。</p> <h3 id="_3-3-第三个参数-增加返回json字符串的可读性-json格式"><a href="#_3-3-第三个参数-增加返回json字符串的可读性-json格式" class="header-anchor">#</a> 3.3 第三个参数 （增加返回JSON字符串的可读性；json格式）</h3> <p><code>JSON.stringify</code>还可以接受第三个参数，用于<strong>增加返回的 JSON 字符串的可读性</strong>。如果是数字，表示每个属性前面添加的空格（最多不超过10个）；如果是字符串（不超过10个字符），则该字符串会添加在每行前面。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">p1</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">p2</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2127. 

  2127. <span class="token comment">/*
    2128. 

  2128. &quot;{
    2129. 

  2129.   &quot;p1&quot;: 1,
    2130. 

  2130.   &quot;p2&quot;: 2
    2131. 

  2131. }&quot;
    2132. 

  2132. */</span>
    2133. 

  2133. <span class="token comment">// 参数三是数值在使用JSON.parse()解析时正常</span>
    2134. 

  2134. 

  2135. 

  2136. 

  2137. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">p1</span><span class="token operator">:</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token literal-property property">p2</span><span class="token operator">:</span><span class="token number">2</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">'|-'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2138. 

  2138. <span class="token comment">/*
    2139. 

  2139. &quot;{
    2140. 

  2140. |-&quot;p1&quot;: 1,
    2141. 

  2141. |-&quot;p2&quot;: 2
    2142. 

  2142. }&quot;
    2143. 

  2143. */</span>
    2144. 

  2144. <span class="token comment">// 参数三是字符串在使用JSON.parse()解析时报错</span>
    2145. 

  2145. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><h3 id="_3-4-参数对象的-tojson-方法-对象自定义tojson方法-转字符串只取该方法返回值"><a href="#_3-4-参数对象的-tojson-方法-对象自定义tojson方法-转字符串只取该方法返回值" class="header-anchor">#</a> 3.4 参数对象的 toJSON 方法（对象自定义toJSON方法，转字符串只取该方法返回值）</h3> <p>如果参数对象有自定义的<code>toJSON</code>方法，那么<code>JSON.stringify</code>会<strong>使用这个方法的返回值作为参数，而忽略原对象的其他属性</strong>。</p> <p>下面是一个普通的对象。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> user <span class="token operator">=</span> <span class="token punctuation">{</span>
    2146. 

  2146.   <span class="token literal-property property">firstName</span><span class="token operator">:</span> <span class="token string">'三'</span><span class="token punctuation">,</span>
    2147. 

  2147.   <span class="token literal-property property">lastName</span><span class="token operator">:</span> <span class="token string">'张'</span><span class="token punctuation">,</span>
    2148. 

  2148. 

  2149.   <span class="token keyword">get</span> <span class="token function">fullName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    2150. 

  2150.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>lastName <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>firstName<span class="token punctuation">;</span>
    2151. 

  2151.   <span class="token punctuation">}</span>
    2152. 

  2152. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    2153. 

  2153. 

  2154. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>user<span class="token punctuation">)</span>
    2155. 

  2155. <span class="token comment">// &quot;{&quot;firstName&quot;:&quot;三&quot;,&quot;lastName&quot;:&quot;张&quot;,&quot;fullName&quot;:&quot;张三&quot;}&quot;</span>
    2156. 

  2156. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>现在，为这个对象加上<code>toJSON</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> user <span class="token operator">=</span> <span class="token punctuation">{</span>
    2157. 

  2157.   <span class="token literal-property property">firstName</span><span class="token operator">:</span> <span class="token string">'三'</span><span class="token punctuation">,</span>
    2158. 

  2158.   <span class="token literal-property property">lastName</span><span class="token operator">:</span> <span class="token string">'张'</span><span class="token punctuation">,</span>
    2159. 

  2159. 

  2160.   <span class="token keyword">get</span> <span class="token function">fullName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    2161. 

  2161.     <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>lastName <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>firstName<span class="token punctuation">;</span>
    2162. 

  2162.   <span class="token punctuation">}</span><span class="token punctuation">,</span>
    2163. 

  2163. 

  2164.   <span class="token function-variable function">toJSON</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 只使用toJSON的返回值作为参数，忽略user对象的其他参数</span>
    2165. 

  2165.     <span class="token keyword">return</span> <span class="token punctuation">{</span>
    2166. 

  2166.       <span class="token literal-property property">name</span><span class="token operator">:</span> <span class="token keyword">this</span><span class="token punctuation">.</span>lastName <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>firstName
    2167. 

  2167.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    2168. 

  2168.   <span class="token punctuation">}</span>
    2169. 

  2169. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    2170. 

  2170. 

  2171. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>user<span class="token punctuation">)</span>
    2172. 

  2172. <span class="token comment">// &quot;{&quot;name&quot;:&quot;张三&quot;}&quot;</span>
    2173. 

  2173. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，<code>JSON.stringify</code>发现参数对象有<code>toJSON</code>方法，就直接使用这个方法的返回值作为参数，而<strong>忽略原对象的其他参数</strong>。</p> <p><code>Date</code>对象就有一个自己的<code>toJSON</code>方法。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> date <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token string">'2015-01-01'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2174. 

  2174. date<span class="token punctuation">.</span><span class="token function">toJSON</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;2015-01-01T00:00:00.000Z&quot;</span>
    2175. 

  2175. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>date<span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;2015-01-01T00:00:00.000Z&quot;&quot;</span>
    2176. 

  2176. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>JSON.stringify</code>发现处理的是<code>Date</code>对象实例，就会调用这个实例对象的<code>toJSON</code>方法，将该方法的返回值作为参数。</p> <p><code>toJSON</code>方法的一个应用是，<strong>将正则对象自动转为字符串</strong>。因为<code>JSON.stringify</code>默认不能转换正则对象，但是设置了<code>toJSON</code>方法以后，就可以转换正则对象了。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
    2177. 

  2177.   <span class="token literal-property property">reg</span><span class="token operator">:</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">foo</span><span class="token regex-delimiter">/</span></span>
    2178. 

  2178. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    2179. 

  2179. 

  2180. <span class="token comment">// 不设置 toJSON 方法时</span>
    2181. 

  2181. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token comment">// &quot;{&quot;reg&quot;:{}}&quot;</span>
    2182. 

  2182. 

  2183. <span class="token comment">// 设置 toJSON 方法时</span>
    2184. 

  2184. <span class="token class-name">RegExp</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>toJSON <span class="token operator">=</span> <span class="token class-name">RegExp</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>toString<span class="token punctuation">;</span>
    2185. 

  2185. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">stringify</span><span class="token punctuation">(</span><span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">foo</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;/foo/&quot;&quot;</span>
    2186. 

  2186. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码在正则对象的原型上面部署了<code>toJSON()</code>方法，将其指向<code>toString()</code>方法，因此转换成 JSON 格式时，正则对象就先调用<code>toJSON()</code>方法转为字符串，然后再被<code>JSON.stringify()</code>方法处理。</p> <h2 id="_4、json-parse"><a href="#_4、json-parse" class="header-anchor">#</a> 4、JSON.parse()</h2> <p><code>JSON.parse</code>方法<strong>用于将 JSON 字符串转换成对应的JSON值</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'{}'</span><span class="token punctuation">)</span> <span class="token comment">// {}</span>
    2187. 

  2187. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'true'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    2188. 

  2188. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'&quot;foo&quot;'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;foo&quot;</span>
    2189. 

  2189. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'[1, 5, &quot;false&quot;]'</span><span class="token punctuation">)</span> <span class="token comment">// [1, 5, &quot;false&quot;]</span>
    2190. 

  2190. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'null'</span><span class="token punctuation">)</span> <span class="token comment">// null</span>
    2191. 

  2191. 

  2192. <span class="token keyword">var</span> o <span class="token operator">=</span> <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'{&quot;name&quot;: &quot;张三&quot;}'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2193. 

  2193. o<span class="token punctuation">.</span>name <span class="token comment">// 张三</span>
    2194. 

  2194. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>如果传入的字符串不是有效的 JSON 格式，<code>JSON.parse</code>方法将报错。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">&quot;'String'&quot;</span><span class="token punctuation">)</span> <span class="token comment">// illegal single quotes</span>
    2195. 

  2195. <span class="token comment">// SyntaxError: Unexpected token ILLEGAL</span>
    2196. 

  2196. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，双引号字符串中是一个单引号字符串，因为单引号字符串不符合 JSON 格式，所以报错。</p> <p>为了<strong>处理解析错误</strong>，可以将<code>JSON.parse</code>方法放在<code>try...catch</code>代码块中。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">try</span> <span class="token punctuation">{</span>
    2197. 

  2197.   <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">&quot;'String'&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2198. 

  2198. <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2199. 

  2199.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'parsing error'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    2200. 

  2200. <span class="token punctuation">}</span>
    2201. 

  2201. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><code>JSON.parse</code>方法<strong>可以接受一个处理函数，作为第二个参数，用法与<code>JSON.stringify</code>方法类似</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span> value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token comment">// 参数：键名、键值</span>
    2202. 

  2202.   <span class="token keyword">if</span> <span class="token punctuation">(</span>key <span class="token operator">===</span> <span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    2203. 

  2203.     <span class="token keyword">return</span> value <span class="token operator">+</span> <span class="token number">10</span><span class="token punctuation">;</span>
    2204. 

  2204.   <span class="token punctuation">}</span>
    2205. 

  2205.   <span class="token keyword">return</span> value<span class="token punctuation">;</span>
    2206. 

  2206. <span class="token punctuation">}</span>
    2207. 

  2207. 

  2208. <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span><span class="token string">'{&quot;a&quot;: 1, &quot;b&quot;: 2}'</span><span class="token punctuation">,</span> f<span class="token punctuation">)</span>
    2209. 

  2209. <span class="token comment">// {a: 11, b: 2}</span>
    2210. 

  2210. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>JSON.parse</code>的第二个参数是一个函数，如果键名是<code>a</code>，该函数会将键值加上10。</p> <h2 id="文档"><a href="#文档" class="header-anchor">#</a> 文档</h2> <p>学习文档：<a href="https://wangdoc.com/javascript/" target="_blank" rel="noopener noreferrer">https://wangdoc.com/javascript/<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《JavaScript教程》笔记/02.内置对象.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=JavaScript" title="标签">#JavaScript</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/0796ba76b4b55368/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">基础</div></a> <a href="/blog/pages/659b5af5e2e704e0/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">面向对象</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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  2212.         <a href="/blog/pages/0796ba76b4b55368/" class="prev">基础</a></span> <span class="next"><a href="/blog/pages/659b5af5e2e704e0/">面向对象</a>→
    2213. 

  2213.       </span></p></div></div></div> <div class="article-list"><div class="article-title"><a href="/blog/archives/" class="iconfont icon-bi">最近更新</a></div> <div class="article-wrapper"><dl><dd>01</dd> <dt><a href="/blog/pages/922650/"><div>
    2214. 

  2214.             Git修改分支名
    2215. 

  2215.             <!----></div></a> <span class="date">08-11</span></dt></dl><dl><dd>02</dd> <dt><a href="/blog/pages/55f894/"><div>
    2216. 

  2216.             CSS给table的tbody添加滚动条
    2217. 

  2217.             <!----></div></a> <span class="date">06-29</span></dt></dl><dl><dd>03</dd> <dt><a href="/blog/pages/829589/"><div>
    2218. 

  2218.             我做了一个手写春联小网页，祝大家虎年暴富
    2219. 

  2219.             <span class="title-tag">
    2220. 

  2220.               原创
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  2221.             </span></div></a> <span class="date">01-28</span></dt></dl> <dl><dd></dd> <dt><a href="/blog/archives/" class="more">更多文章&gt;</a></dt></dl></div></div></main></div> <div class="footer"><div class="icons"><a href="mailto:30363811@qq.com" title="发邮件" target="_blank" class="iconfont icon-youjian"></a><a href="https://github.com/heBody" title="GitHub" target="_blank" class="iconfont icon-github"></a></div> 
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