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  19.   <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></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/blog/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/note/javascript/" class="nav-link">《JavaScript教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/js/" class="nav-link">《JavaScript高级程序设计》</a></li><li class="dropdown-subitem"><a href="/blog/note/es6/" class="nav-link">《ES6 教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/vue/" class="nav-link">《Vue》</a></li><li class="dropdown-subitem"><a href="/blog/note/react/" class="nav-link">《React》</a></li><li class="dropdown-subitem"><a href="/blog/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/blog/note/git/" class="nav-link">《Git》</a></li><li class="dropdown-subitem"><a href="/blog/pages/51afd6/" class="nav-link">TypeScript</a></li><li class="dropdown-subitem"><a href="/blog/pages/4643cd/" class="nav-link">JS设计模式总结</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/blog/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/blog/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="更多" class="dropdown-title"><a href="/blog/more/" class="link-title">更多</a> <span class="title" style="display:none;">更多</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/f2a556/" class="nav-link">学习</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/aea6571b7a8bae86/" class="nav-link">面试</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/2d615df9a36a98ed/" class="nav-link">心情杂货</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/baaa02/" class="nav-link">实用技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/friends/" class="nav-link">友情链接</a></li></ul></div></div><div class="nav-item"><a href="/blog/about/" class="nav-link">关于</a></div><div class="nav-item"><a href="/blog/pages/beb6c0bd8a66cea6/" class="nav-link">收藏</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="索引" class="dropdown-title"><a href="/blog/archives/" class="link-title">索引</a> <span class="title" style="display:none;">索引</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/categories/" class="nav-link">分类</a></li><li class="dropdown-item"><!----> <a href="/blog/tags/" class="nav-link">标签</a></li><li class="dropdown-item"><!----> <a href="/blog/archives/" class="nav-link">归档</a></li></ul></div></div> <a href="https://github.com/heBody/blog" target="_blank" rel="noopener noreferrer" class="repo-link">
    20. 

  20.     GitHub
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  21.     <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></div></header> <div class="sidebar-mask"></div> <div class="sidebar-hover-trigger"></div> <aside class="sidebar" style="display:none;"><div class="blogger"><img src="/blog/img/head.jpg"> <div class="blogger-info"><h3>彪哥</h3> <span>爱好前端</span></div></div> <nav class="nav-links"><div class="nav-item"><a href="/blog/" class="nav-link">首页</a></div><div class="nav-item"><a href="http://fseller.com" target="_blank" rel="noopener noreferrer" class="nav-link external">
    22. 

  22.   个人游戏网站
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  23.   <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></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/blog/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/note/javascript/" class="nav-link">《JavaScript教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/js/" class="nav-link">《JavaScript高级程序设计》</a></li><li class="dropdown-subitem"><a href="/blog/note/es6/" class="nav-link">《ES6 教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/vue/" class="nav-link">《Vue》</a></li><li class="dropdown-subitem"><a href="/blog/note/react/" class="nav-link">《React》</a></li><li class="dropdown-subitem"><a href="/blog/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/blog/note/git/" class="nav-link">《Git》</a></li><li class="dropdown-subitem"><a href="/blog/pages/51afd6/" class="nav-link">TypeScript</a></li><li class="dropdown-subitem"><a href="/blog/pages/4643cd/" class="nav-link">JS设计模式总结</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/blog/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/blog/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="更多" class="dropdown-title"><a href="/blog/more/" class="link-title">更多</a> <span class="title" style="display:none;">更多</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/f2a556/" class="nav-link">学习</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/aea6571b7a8bae86/" class="nav-link">面试</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/2d615df9a36a98ed/" class="nav-link">心情杂货</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/baaa02/" class="nav-link">实用技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/friends/" class="nav-link">友情链接</a></li></ul></div></div><div class="nav-item"><a href="/blog/about/" class="nav-link">关于</a></div><div class="nav-item"><a href="/blog/pages/beb6c0bd8a66cea6/" class="nav-link">收藏</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="索引" class="dropdown-title"><a href="/blog/archives/" class="link-title">索引</a> <span class="title" style="display:none;">索引</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/categories/" class="nav-link">分类</a></li><li class="dropdown-item"><!----> <a href="/blog/tags/" class="nav-link">标签</a></li><li class="dropdown-item"><!----> <a href="/blog/archives/" class="nav-link">归档</a></li></ul></div></div> <a href="https://github.com/heBody/blog" target="_blank" rel="noopener noreferrer" class="repo-link">
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  24.     GitHub
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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/" aria-current="page" class="active sidebar-link">基础</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/0796ba76b4b55368/#一、js的一些名词概念" class="sidebar-link">一、JS的一些名词概念</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是作用域" class="sidebar-link">什么是作用域？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是闭包" class="sidebar-link">什么是闭包？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是构造函数" class="sidebar-link">什么是构造函数？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是实例对象" class="sidebar-link">什么是实例对象？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是this" class="sidebar-link">什么是this？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是原型" class="sidebar-link">什么是原型？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是原型链" class="sidebar-link">什么是原型链？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是constructor" class="sidebar-link">什么是constructor？</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#什么是包装对象" class="sidebar-link">什么是包装对象？</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/0796ba76b4b55368/#二、数据类型的转换" class="sidebar-link">二、数据类型的转换</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_1、强制-手动-转换" class="sidebar-link">1、强制（手动）转换</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#number" class="sidebar-link">Number()</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#string" class="sidebar-link">String()</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#boolean" class="sidebar-link">Boolean()</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_2、自动转换" class="sidebar-link">2、自动转换</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#自动转换为布尔值-boolean" class="sidebar-link">自动转换为布尔值（Boolean）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#自动转换为字符串-string" class="sidebar-link">自动转换为字符串（String）</a></li><li class="sidebar-sub-header level5"><a href="/blog/pages/0796ba76b4b55368/#所有类型的值与字符串相加都会变成字符串" class="sidebar-link">所有类型的值与字符串相加都会变成字符串</a></li><li class="sidebar-sub-header level5"><a href="/blog/pages/0796ba76b4b55368/#" class="sidebar-link"></a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#自动转换为数值-number" class="sidebar-link">自动转换为数值（Number）</a></li><li class="sidebar-sub-header level5"><a href="/blog/pages/0796ba76b4b55368/#除加号与字符串运行会转成字符串外-其他运行基本都会自动转成数值" class="sidebar-link">除加号与字符串运行会转成字符串外，其他运行基本都会自动转成数值</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/0796ba76b4b55368/#三、错误处理机制" class="sidebar-link">三、错误处理机制</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_1、error实例对象" class="sidebar-link">1、Error实例对象</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_2、原生错误类型" class="sidebar-link">2、原生错误类型</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#syntaxerror-对象-语法错误" class="sidebar-link">SyntaxError 对象（语法错误）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#referenceerror-对象-引用错误" class="sidebar-link">ReferenceError 对象（引用错误）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#rangeerror-对象-范围错误" class="sidebar-link">RangeError 对象（范围错误）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#typeerror-对象-类型错误" class="sidebar-link">TypeError 对象（类型错误）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#urierror-对象-uri错误" class="sidebar-link">URIError 对象 （URI错误）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#evalerror-对象-eval错误" class="sidebar-link">EvalError 对象 （eval错误）</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/0796ba76b4b55368/#总结-开发者手动使用" class="sidebar-link">总结（开发者手动使用）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_3、自定义错误" class="sidebar-link">3、自定义错误</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_4、throw-语句-中断程序并抛出错误" class="sidebar-link">4、throw 语句 （中断程序并抛出错误）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_5、try-catch-结构-捕获错误-对错误进行处理-不中断" class="sidebar-link">5、try...catch 结构（捕获错误，对错误进行处理，不中断）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#_6、finally-代码块-在try-catch最后-必执行的" class="sidebar-link">6、finally 代码块（在try..catch最后，必执行的）</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/0796ba76b4b55368/#四、console对象" class="sidebar-link">四、console对象</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/0796ba76b4b55368/#console-time-console-timeend-用于计算程序的用时" class="sidebar-link">console.time()，console.timeEnd() （用于计算程序的用时）</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/0796ba76b4b55368/#文档" class="sidebar-link">文档</a></li></ul></li><li><a href="/blog/pages/74d2ab3fbfeaaa68/" class="sidebar-link">内置对象</a></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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">基础<!----></h1>  <div class="theme-vdoing-content content__default"><blockquote><p>说明：本章内容为博主在原教程基础上添加自己的学习笔记，来源<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>，教程版权归原作者所有。</p></blockquote> <h1 id="基础篇"><a href="#基础篇" class="header-anchor">#</a> 基础篇</h1> <h2 id="一、js的一些名词概念"><a href="#一、js的一些名词概念" class="header-anchor">#</a> 一、JS的一些名词概念</h2> <h4 id="什么是作用域"><a href="#什么是作用域" class="header-anchor">#</a> 什么是作用域？</h4> <p>变量存在的范围。</p> <p>可分为全局作用域和函数作用域，ES6新增块级作用域。</p> <h4 id="什么是闭包"><a href="#什么是闭包" class="header-anchor">#</a> 什么是闭包？</h4> <p>闭包就是能够读取其他函数内部变量的函数。</p> <ul><li>闭包的形式：函数内部定义函数</li> <li>本质上闭包就是将函数内部和外部连接起来的一座桥梁</li></ul> <p>闭包的作用：</p> <ul><li>可以读取函数内部变量</li> <li>让这些变量始终保持在内存中，即闭包可以使得它诞生的环境一直存在。</li> <li>封装对象的私有属性和私有方法</li></ul> <h4 id="什么是构造函数"><a href="#什么是构造函数" class="header-anchor">#</a> 什么是构造函数？</h4> <p>用于构造(生成)实例的一个函数，使实例拥有构造函数内定于的属性和方法。</p> <h4 id="什么是实例对象"><a href="#什么是实例对象" class="header-anchor">#</a> 什么是实例对象？</h4> <p>实例对象就是通过new 构造函数生成的，拥有构造函数内定于的属性和方法的一个对象。</p> <h4 id="什么是this"><a href="#什么是this" class="header-anchor">#</a> 什么是this？</h4> <p>就是属性或方法当前所在的对象，指向当前运行环境（对象）</p> <h4 id="什么是原型"><a href="#什么是原型" class="header-anchor">#</a> 什么是原型？</h4> <p>每个函数都有一个prototype属性，指向一个对象，该对象称为原型对象。</p> <h4 id="什么是原型链"><a href="#什么是原型链" class="header-anchor">#</a> 什么是原型链？</h4> <p>所有对象都有自己的原型对象，由于原型对象也是对象，因此它也有自己的原型，这就会形成一个原型链。</p> <p>最顶层的原型是Object.prototype。</p> <blockquote><p>读取对象属性时，JS会先在对象自身上找，找到就直接返回，如果找不到，会到原型上找，如果还是找不到，就会去原型的原型上找，最终会到最顶层的Object.prototype上找，还是找不到就会返回undefined。</p></blockquote> <h4 id="什么是constructor"><a href="#什么是constructor" class="header-anchor">#</a> 什么是constructor？</h4> <p>prototype原型对象都有一个constructor属性，默认指向prototype对象所在的构造函数。</p> <h4 id="什么是包装对象"><a href="#什么是包装对象" class="header-anchor">#</a> 什么是包装对象？</h4> <p>包装对象指的是将原始类型（数字、字符串、布尔值）进行实例化。</p> <h2 id="二、数据类型的转换"><a href="#二、数据类型的转换" class="header-anchor">#</a> 二、数据类型的转换</h2> <h3 id="_1、强制-手动-转换"><a href="#_1、强制-手动-转换" class="header-anchor">#</a> 1、强制（手动）转换</h3> <p>强制转换主要指使用<code>Number()</code>、<code>String()</code>和<code>Boolean()</code>三个函数，手动将各种类型的值，分别转换成<strong>数字、字符串、布尔值</strong>。</p> <h4 id="number"><a href="#number" class="header-anchor">#</a> Number()</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 数值：转换后还是原来的值</span>
    26. 

  26. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token number">324</span><span class="token punctuation">)</span> <span class="token comment">// 324</span>
    27. 

  27. 

  28. <span class="token comment">// 字符串：如果可以被解析为数值，则转换为相应的数值</span>
    29. 

  29. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'324'</span><span class="token punctuation">)</span> <span class="token comment">// 324</span>
    30. 

  30. 

  31. <span class="token comment">// 字符串：如果不可以被解析为数值，返回 NaN</span>
    32. 

  32. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'324abc'</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    33. 

  33. 

  34. <span class="token comment">// 空字符串转为0</span>
    35. 

  35. <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>
    36. 

  36. 

  37. <span class="token comment">// 布尔值：true 转成 1，false 转成 0</span>
    38. 

  38. <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>
    39. 

  39. <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>
    40. 

  40. 

  41. <span class="token comment">// undefined：转成 NaN</span>
    42. 

  42. <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>
    43. 

  43. 

  44. <span class="token comment">// null：转成0</span>
    45. 

  45. <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>
    46. 

  46. 

  47. <span class="token function">Number</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">// NaN</span>
    48. 

  48. <span class="token function">Number</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 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 comment">// NaN</span>
    49. 

  49. <span class="token function">Number</span><span class="token punctuation">(</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">// 5</span>
    50. 

  50. <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>
    51. 

  51. 

  52. <span class="token comment">//使用parseInt()转数组</span>
    53. 

  53. <span class="token function">parseInt</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 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 comment">// 1</span>
    54. 

  54. </code></pre> <div class="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></div></div><h4 id="string"><a href="#string" class="header-anchor">#</a> String()</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 原始类型的转换</span>
    55. 

  55. <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>
    56. 

  56. <span class="token function">String</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;abc&quot;</span>
    57. 

  57. <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>
    58. 

  58. <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">// &quot;undefined&quot;</span>
    59. 

  59. <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">// &quot;null&quot;</span>
    60. 

  60. 

  61. <span class="token comment">// 对象的转换</span>
    62. 

  62. <span class="token function">String</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">// &quot;[object Object]&quot;</span>
    63. 

  63. <span class="token function">String</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 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 comment">// &quot;1,2,3&quot;</span>
    64. 

  64. <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">// &quot;&quot;  空字符串</span>
    65. 

  65. <span class="token function">String</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">// &quot;function(){}&quot;</span>
    66. 

  66. </code></pre> <div class="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><h4 id="boolean"><a href="#boolean" class="header-anchor">#</a> Boolean()</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 除了这五个为false，其他都为true</span>
    67. 

  67. <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>
    68. 

  68. <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>
    69. 

  69. <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>
    70. 

  70. <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>
    71. 

  71. <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>
    72. 

  72. 

  73. <span class="token comment">//true</span>
    74. 

  74. <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>
    75. 

  75. <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>
    76. 

  76. <span class="token function">Boolean</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 comment">// true</span>
    77. 

  77. 

  78. <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>
    79. 

  79. <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">// true // 注意字符串内有个空格</span>
    80. 

  80. </code></pre> <div class="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="_2、自动转换"><a href="#_2、自动转换" class="header-anchor">#</a> 2、自动转换</h3> <p>下面介绍自动转换，它是<strong>以强制转换为基础的</strong>。</p> <p>遇到以下三种情况时，JavaScript 会自动转换数据类型，即转换是自动完成的，用户不可见。</p> <p>第一种情况，<strong>不同类型的数据互相运算</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token number">123</span> <span class="token operator">+</span> <span class="token string">'abc'</span> <span class="token comment">// &quot;123abc&quot;</span>
    81. 

  81. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>第二种情况，<strong>对非布尔值类型的数据求布尔值</strong>。</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 string">'abc'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    82. 

  82.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span>
    83. 

  83. <span class="token punctuation">}</span>  <span class="token comment">// &quot;hello&quot;</span>
    84. 

  84. </code></pre> <div class="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>第三种情况，<strong>对非数值类型的值使用一元运算符（即<code>+</code>和<code>-</code>）</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token operator">+</span> <span class="token punctuation">{</span><span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'bar'</span><span class="token punctuation">}</span> <span class="token comment">// NaN</span>
    85. 

  85. <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 comment">// NaN</span>
    86. 

  86. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>自动转换的规则是这样的：预期什么类型的值，就调用该类型的转换函数。比如，某个位置预期为字符串，就调用<code>String</code>函数进行转换。如果该位置即可以是字符串，也可能是数值，那么默认转为数值。</p> <p>由于自动转换具有不确定性，而且不易除错，建议在预期为布尔值、数值、字符串的地方，全部使用<code>Boolean</code>、<code>Number</code>和<code>String</code>函数进行显式转换。</p> <h4 id="自动转换为布尔值-boolean"><a href="#自动转换为布尔值-boolean" class="header-anchor">#</a> 自动转换为布尔值（Boolean）</h4> <p>JavaScript 遇到预期为布尔值的地方（比如<code>if</code>语句的条件部分），就会将非布尔值的参数自动转换为布尔值。系统内部会自动调用<code>Boolean</code>函数。</p> <p>因此除了以下五个值，其他都是自动转为<code>true</code>。</p> <ul><li><code>undefined</code></li> <li><code>null</code></li> <li><code>+0</code>或<code>-0</code></li> <li><code>NaN</code></li> <li><code>''</code>（空字符串）</li></ul> <p>下面这个例子中，条件部分的每个值都相当于<code>false</code>，使用否定运算符后，就变成了<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 operator">!</span><span class="token keyword">undefined</span>
    87. 

  87.   <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span><span class="token keyword">null</span>
    88. 

  88.   <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span><span class="token number">0</span>
    89. 

  89.   <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span><span class="token number">NaN</span>
    90. 

  90.   <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span><span class="token string">''</span>
    91. 

  91. <span class="token punctuation">)</span> <span class="token punctuation">{</span>
    92. 

  92.   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>
    93. 

  93. <span class="token punctuation">}</span> <span class="token comment">// true</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><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>下面两种写法，有时也用于将一个表达式转为布尔值。它们内部调用的也是<code>Boolean</code>函数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 三元运算符</span>
    95. 

  95. expression <span class="token operator">?</span> <span class="token boolean">true</span> <span class="token operator">:</span> <span class="token boolean">false</span>
    96. 

  96. 

  97. <span class="token comment">// 取反运算符</span>
    98. 

  98. <span class="token operator">!</span><span class="token operator">!</span> expression
    99. 

  99. </code></pre> <div class="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="自动转换为字符串-string"><a href="#自动转换为字符串-string" class="header-anchor">#</a> 自动转换为字符串（String）</h4> <p>JavaScript 遇到预期为字符串的地方，就会将非字符串的值自动转为字符串。<strong>具体规则是，先将复合类型的值转为原始类型的值，再将原始类型的值转为字符串</strong>。</p> <p>字符串的自动转换，<strong>主要发生在字符串的加法运算时</strong>。当一个值为字符串，另一个值为非字符串，则后者转为字符串。</p> <h5 id="所有类型的值与字符串相加都会变成字符串"><a href="#所有类型的值与字符串相加都会变成字符串" class="header-anchor">#</a> 所有类型的值与字符串相加都会变成字符串</h5> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'5'</span> <span class="token operator">+</span> <span class="token number">1</span> <span class="token comment">// '51'</span>
    100. 

  100.  <span class="token number">1</span>  <span class="token operator">+</span> <span class="token string">'5'</span> <span class="token comment">// '15'</span>
    101. 

  101. <span class="token string">'5'</span> <span class="token operator">+</span> <span class="token boolean">true</span> <span class="token comment">// &quot;5true&quot;</span>
    102. 

  102. <span class="token string">'5'</span> <span class="token operator">+</span> <span class="token boolean">false</span> <span class="token comment">// &quot;5false&quot;</span>
    103. 

  103. <span class="token string">'5'</span> <span class="token operator">+</span> <span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token comment">// &quot;5[object Object]&quot;</span>
    104. 

  104.  <span class="token number">5</span> <span class="token operator">+</span> <span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token comment">// &quot;5[object Object]&quot;</span>
    105. 

  105. <span class="token string">'5'</span> <span class="token operator">+</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token comment">// &quot;5&quot;</span>
    106. 

  106.  <span class="token number">5</span> <span class="token operator">+</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token comment">// &quot;5&quot;</span>
    107. 

  107. <span class="token string">'5'</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">// &quot;5function (){}&quot;</span>
    108. 

  108. <span class="token string">'5'</span> <span class="token operator">+</span> <span class="token keyword">undefined</span> <span class="token comment">// &quot;5undefined&quot;</span>
    109. 

  109. <span class="token string">'5'</span> <span class="token operator">+</span> <span class="token keyword">null</span> <span class="token comment">// &quot;5null&quot;</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><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><h5 id=""><a href="#" class="header-anchor">#</a></h5> <p>这种自动转换不注意的话很容易出错。</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>
    111. 

  111.   <span class="token literal-property property">width</span><span class="token operator">:</span> <span class="token string">'100'</span>
    112. 

  112. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    113. 

  113. 

  114. obj<span class="token punctuation">.</span>width <span class="token operator">+</span> <span class="token number">20</span> <span class="token comment">// &quot;10020&quot;</span>
    115. 

  115. <span class="token function">parerInt</span><span class="token punctuation">(</span>obj<span class="token punctuation">.</span>width<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">20</span> <span class="token comment">// 120</span>
    116. 

  116. </code></pre> <div class="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，开发者可能期望返回<code>120</code>，但是由于自动转换，实际上返回了一个字符<code>10020</code>。正确做法是先把字符串转成数字。</p> <h4 id="自动转换为数值-number"><a href="#自动转换为数值-number" class="header-anchor">#</a> 自动转换为数值（Number）</h4> <p>JavaScript 遇到预期为数值的地方，就会将参数值自动转换为数值。系统内部会自动调用<code>Number</code>函数。</p> <h5 id="除加号与字符串运行会转成字符串外-其他运行基本都会自动转成数值"><a href="#除加号与字符串运行会转成字符串外-其他运行基本都会自动转成数值" class="header-anchor">#</a> 除加号与字符串运行会转成字符串外，其他运行基本都会自动转成数值</h5> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token string">'5'</span> <span class="token operator">-</span> <span class="token string">'2'</span> <span class="token comment">// 3</span>
    117. 

  117. <span class="token string">'5'</span> <span class="token operator">*</span> <span class="token string">'2'</span> <span class="token comment">// 10</span>
    118. 

  118. <span class="token boolean">true</span> <span class="token operator">-</span> <span class="token number">1</span>  <span class="token comment">// 0</span>
    119. 

  119. <span class="token boolean">false</span> <span class="token operator">-</span> <span class="token number">1</span> <span class="token comment">// -1</span>
    120. 

  120. <span class="token string">'1'</span> <span class="token operator">-</span> <span class="token number">1</span>   <span class="token comment">// 0</span>
    121. 

  121. <span class="token string">'5'</span> <span class="token operator">*</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>    <span class="token comment">// 0</span>
    122. 

  122. <span class="token boolean">false</span> <span class="token operator">/</span> <span class="token string">'5'</span> <span class="token comment">// 0</span>
    123. 

  123. <span class="token string">'abc'</span> <span class="token operator">-</span> <span class="token number">1</span>   <span class="token comment">// NaN</span>
    124. 

  124. <span class="token keyword">null</span> <span class="token operator">+</span> <span class="token number">1</span> <span class="token comment">// 1</span>
    125. 

  125. <span class="token keyword">undefined</span> <span class="token operator">+</span> <span class="token number">1</span> <span class="token comment">// NaN</span>
    126. 

  126. 

  127. <span class="token boolean">true</span><span class="token operator">+</span><span class="token boolean">true</span> <span class="token comment">// 2</span>
    128. 

  128. </code></pre> <div class="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，运算符两侧的运算子，都被转成了数值。</p> <blockquote><p>注意：<code>null</code>转为数值时为<code>0</code>，而<code>undefined</code>转为数值时为<code>NaN</code>。</p></blockquote> <p><strong>数值与布尔值、null也会转为数值</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token number">5</span><span class="token operator">+</span><span class="token boolean">true</span> <span class="token comment">// 6</span>
    129. 

  129. <span class="token number">5</span><span class="token operator">+</span><span class="token boolean">false</span> <span class="token comment">// 5</span>
    130. 

  130. <span class="token number">5</span><span class="token operator">+</span><span class="token keyword">null</span> <span class="token comment">//5</span>
    131. 

  131. </code></pre> <div class="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>一元运算符也会把运算子转成数值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token operator">+</span><span class="token string">'abc'</span> <span class="token comment">// NaN</span>
    132. 

  132. <span class="token operator">-</span><span class="token string">'abc'</span> <span class="token comment">// NaN</span>
    133. 

  133. <span class="token operator">+</span><span class="token boolean">true</span> <span class="token comment">// 1</span>
    134. 

  134. <span class="token operator">-</span><span class="token boolean">false</span> <span class="token comment">// 0</span>
    135. 

  135. </code></pre> <div class="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="三、错误处理机制"><a href="#三、错误处理机制" class="header-anchor">#</a> 三、错误处理机制</h2> <h3 id="_1、error实例对象"><a href="#_1、error实例对象" class="header-anchor">#</a> 1、Error实例对象</h3> <p>JavaScript 解析或运行时，一旦发生错误，引擎就会抛出一个错误对象。JavaScript 原生提供<code>Error</code>构造函数，所有抛出的错误都是这个构造函数的实例。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> err <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    136. 

  136. err<span class="token punctuation">.</span>message <span class="token comment">// &quot;出错了&quot;</span>
    137. 

  137. </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，我们调用<code>Error</code>构造函数，生成一个实例对象<code>err</code>。<code>Error</code>构造函数接受一个参数，表示错误提示，可以从实例的<code>message</code>属性读到这个参数。<strong>抛出<code>Error</code>实例对象以后，整个程序就中断在发生错误的地方，不再往下执行</strong>。</p> <p>JavaScript 语言标准只提到，<code>Error</code>实例对象必须有<code>message</code>属性，表示出错时的提示信息，没有提到其他属性。大多数 JavaScript 引擎，对<code>Error</code>实例还提供<code>name</code>和<code>stack</code>属性，分别表示错误的名称和错误的堆栈，但它们是非标准的，不是每种实现都有。</p> <ul><li><strong>message</strong>：错误提示信息</li> <li><strong>name</strong>：错误名称（非标准属性）</li> <li><strong>stack</strong>：错误的堆栈（非标准属性）</li></ul> <p>使用<code>name</code>和<code>message</code>这两个属性，可以对发生什么错误有一个大概的了解。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> err <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    138. 

  138. <span class="token keyword">if</span> <span class="token punctuation">(</span>err<span class="token punctuation">.</span>name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    139. 

  139.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>err<span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">': '</span> <span class="token operator">+</span> err<span class="token punctuation">.</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Error: 出错了</span>
    140. 

  140. <span class="token punctuation">}</span>
    141. 

  141. </code></pre> <div class="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>stack</code>属性用来查看错误发生时的堆栈。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">throwit</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    142. 

  142.   <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">''</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    143. 

  143. <span class="token punctuation">}</span>
    144. 

  144. 

  145. <span class="token keyword">function</span> <span class="token function">catchit</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    146. 

  146.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    147. 

  147.     <span class="token function">throwit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    148. 

  148.   <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>
    149. 

  149.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>stack<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// print stack trace</span>
    150. 

  150.   <span class="token punctuation">}</span>
    151. 

  151. <span class="token punctuation">}</span>
    152. 

  152. 

  153. <span class="token function">catchit</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    154. 

  154. <span class="token comment">// Error</span>
    155. 

  155. <span class="token comment">//    at throwit (~/examples/throwcatch.js:9:11) // 堆栈的最内层throwit函数</span>
    156. 

  156. <span class="token comment">//    at catchit (~/examples/throwcatch.js:3:9) // 向外一层catchit函数</span>
    157. 

  157. <span class="token comment">//    at repl:1:5 // 函数的运行环境</span>
    158. 

  158. 

  159. 

  160. <span class="token comment">// 堆栈信息说明</span>
    161. 

  161. <span class="token comment">// Error 错误</span>
    162. 

  162. <span class="token comment">//    at throwit 在throwit方法 (~/examples/throwcatch.js:9:11) 文件名：第几行：第几个字符</span>
    163. 

  163. <span class="token comment">//    at catchit 在catchit方法 (~/examples/throwcatch.js:3:9) 文件名：第几行：第几个字符</span>
    164. 

  164. <span class="token comment">//    at repl:1:5 // 函数的运行环境</span>
    165. 

  165. </code></pre> <div class="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，错误堆栈的最内层是<code>throwit</code>函数，然后是<code>catchit</code>函数，最后是函数的运行环境。</p> <h3 id="_2、原生错误类型"><a href="#_2、原生错误类型" class="header-anchor">#</a> 2、原生错误类型</h3> <p><code>Error</code>实例对象是最一般的错误类型，在它的基础上，JavaScript 还定义了其他6种错误对象。也就是说，存在<code>Error</code>的6个派生对象。</p> <h4 id="syntaxerror-对象-语法错误"><a href="#syntaxerror-对象-语法错误" class="header-anchor">#</a> SyntaxError 对象（语法错误）</h4> <p><code>SyntaxError</code>对象是解析代码时发生的<strong>语法错误</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 变量名错误</span>
    166. 

  166. <span class="token keyword">var</span> 1a<span class="token punctuation">;</span>
    167. 

  167. <span class="token comment">// Uncaught SyntaxError: Invalid or unexpected token</span>
    168. 

  168. <span class="token comment">// 语法错误：无效或意外的标记符号</span>
    169. 

  169. 

  170. <span class="token comment">// 缺少括号</span>
    171. 

  171. console<span class="token punctuation">.</span>log <span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    172. 

  172. <span class="token comment">// Uncaught SyntaxError: Unexpected string</span>
    173. 

  173. <span class="token comment">// 语法错误：意外的字符串</span>
    174. 

  174. </code></pre> <div class="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>上面代码的错误，都是在语法解析阶段就可以发现，所以会抛出<code>SyntaxError</code>。第一个错误提示是“token 非法”，第二个错误提示是“字符串不符合要求”。</p> <h4 id="referenceerror-对象-引用错误"><a href="#referenceerror-对象-引用错误" class="header-anchor">#</a> ReferenceError 对象（引用错误）</h4> <p><code>ReferenceError</code>对象是<strong>引用一个不存在的变量时发生的错误</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 使用一个不存在的变量</span>
    175. 

  175. unknownVariable
    176. 

  176. <span class="token comment">// Uncaught ReferenceError: unknownVariable is not defined</span>
    177. 

  177. <span class="token comment">// 引用错误： unknownVariable 没有定义</span>
    178. 

  178. </code></pre> <div class="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>另一种触发场景是，将一个值分配给无法分配的对象，比如对函数的运行结果或者<code>this</code>赋值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 等号左侧不是变量</span>
    179. 

  179. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token number">1</span>
    180. 

  180. <span class="token comment">// Uncaught ReferenceError: Invalid left-hand side in assignment</span>
    181. 

  181. <span class="token comment">// 引用错误： 赋值中左边是无效的</span>
    182. 

  182. 

  183. <span class="token comment">// this 对象不能手动赋值</span>
    184. 

  184. <span class="token keyword">this</span> <span class="token operator">=</span> <span class="token number">1</span>
    185. 

  185. <span class="token comment">// ReferenceError: Invalid left-hand side in assignment</span>
    186. 

  186. <span class="token comment">// 引用错误：  赋值中左边是无效的</span>
    187. 

  187. </code></pre> <div class="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>上面代码对函数<code>console.log</code>的运行结果和<code>this</code>赋值，结果都引发了<code>ReferenceError</code>错误。</p> <h4 id="rangeerror-对象-范围错误"><a href="#rangeerror-对象-范围错误" class="header-anchor">#</a> RangeError 对象（范围错误）</h4> <p><code>RangeError</code>对象是一个值<strong>超出有效范围时发生的错误</strong>。主要有几种情况，一是数组长度为负数，二是<code>Number</code>对象的方法参数超出范围，以及函数堆栈超过最大值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 数组长度不得为负数</span>
    188. 

  188. <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">1</span><span class="token punctuation">)</span>
    189. 

  189. <span class="token comment">// Uncaught RangeError: Invalid array length</span>
    190. 

  190. <span class="token comment">// 范围错误：无效的数组长度</span>
    191. 

  191. </code></pre> <div class="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="typeerror-对象-类型错误"><a href="#typeerror-对象-类型错误" class="header-anchor">#</a> TypeError 对象（类型错误）</h4> <p><code>TypeError</code>对象是<strong>变量或参数不是预期类型时发生的错误</strong>。比如，对字符串、布尔值、数值等原始类型的值使用<code>new</code>命令，就会抛出这种错误，因为<code>new</code>命令的参数应该是一个构造函数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">new</span> <span class="token class-name">123</span>
    192. 

  192. <span class="token comment">// Uncaught TypeError: number is not a function</span>
    193. 

  193. <span class="token comment">// 类型错误： 数字不是一个函数</span>
    194. 

  194. 

  195. <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>
    196. 

  196. obj<span class="token punctuation">.</span><span class="token function">unknownMethod</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    197. 

  197. <span class="token comment">// Uncaught TypeError: obj.unknownMethod is not a function</span>
    198. 

  198. <span class="token comment">// 类型错误：obj.unknownMethod 不是一个函数</span>
    199. 

  199. </code></pre> <div class="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>上面代码的第二种情况，调用对象不存在的方法，也会抛出<code>TypeError</code>错误，因为<code>obj.unknownMethod</code>的值是<code>undefined</code>，而不是一个函数。</p> <h4 id="urierror-对象-uri错误"><a href="#urierror-对象-uri错误" class="header-anchor">#</a> URIError 对象 （URI错误）</h4> <p><code>URIError</code>对象是 <strong>URI 相关函数的参数不正确时抛出的错误</strong>，主要涉及<code>encodeURI()</code>、<code>decodeURI()</code>、<code>encodeURIComponent()</code>、<code>decodeURIComponent()</code>、<code>escape()</code>和<code>unescape()</code>这六个函数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">decodeURI</span><span class="token punctuation">(</span><span class="token string">'%2'</span><span class="token punctuation">)</span>
    200. 

  200. <span class="token comment">// URIError: URI malformed</span>
    201. 

  201. </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="evalerror-对象-eval错误"><a href="#evalerror-对象-eval错误" class="header-anchor">#</a> EvalError 对象 （eval错误）</h4> <p><code>eval</code>函数没有被正确执行时，会抛出<code>EvalError</code>错误。该<strong>错误类型已经不再使用了，只是为了保证与以前代码兼容，才继续保留。</strong></p> <h4 id="总结-开发者手动使用"><a href="#总结-开发者手动使用" class="header-anchor">#</a> 总结（开发者手动使用）</h4> <p>以上这6种派生错误，连同原始的<code>Error</code>对象，都是构造函数。<strong>开发者可以使用它们，手动生成错误对象的实例</strong>。这些构造函数都接受一个参数，代表错误提示信息（message）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> err1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'出错了！'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    202. 

  202. <span class="token keyword">var</span> err2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">RangeError</span><span class="token punctuation">(</span><span class="token string">'出错了，变量超出有效范围！'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    203. 

  203. <span class="token keyword">var</span> err3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">TypeError</span><span class="token punctuation">(</span><span class="token string">'出错了，变量类型无效！'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    204. 

  204. 

  205. err1<span class="token punctuation">.</span>message <span class="token comment">// &quot;出错了！&quot;</span>
    206. 

  206. err2<span class="token punctuation">.</span>message <span class="token comment">// &quot;出错了，变量超出有效范围！&quot;</span>
    207. 

  207. err3<span class="token punctuation">.</span>message <span class="token comment">// &quot;出错了，变量类型无效！&quot;</span>
    208. 

  208. </code></pre> <div class="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="_3、自定义错误"><a href="#_3、自定义错误" class="header-anchor">#</a> 3、自定义错误</h3> <p>除了 JavaScript 原生提供的七种错误对象，还可以定义自己的错误对象。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">UserError</span><span class="token punctuation">(</span><span class="token parameter">message</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    209. 

  209.   <span class="token keyword">this</span><span class="token punctuation">.</span>message <span class="token operator">=</span> message <span class="token operator">||</span> <span class="token string">'默认信息'</span><span class="token punctuation">;</span>
    210. 

  210.   <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">'UserError'</span><span class="token punctuation">;</span>
    211. 

  211. <span class="token punctuation">}</span>
    212. 

  212. 

  213. <span class="token class-name">UserError</span><span class="token punctuation">.</span>prototype <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 原型继承 Error对象</span>
    214. 

  214. <span class="token class-name">UserError</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>constructor <span class="token operator">=</span> UserError<span class="token punctuation">;</span>
    215. 

  215. 

  216. <span class="token comment">// 使用</span>
    217. 

  217. <span class="token keyword">new</span> <span class="token class-name">UserError</span><span class="token punctuation">(</span><span class="token string">'这是自定义的错误！'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    218. 

  218. </code></pre> <div class="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>上面代码自定义一个错误对象<code>UserError</code>，让它继承<code>Error</code>对象。然后，就可以生成这种自定义类型的错误了。</p> <h3 id="_4、throw-语句-中断程序并抛出错误"><a href="#_4、throw-语句-中断程序并抛出错误" class="header-anchor">#</a> 4、throw 语句 （中断程序并抛出错误）</h3> <p><code>throw</code>语句的作用是<strong>手动中断程序执行，抛出一个错误</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> x <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    219. 

  219. <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">&lt;=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    220. 

  220.   <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">'x 必须为正数'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    221. 

  221. <span class="token punctuation">}</span>
    222. 

  222. <span class="token comment">// Uncaught Error: x 必须为正数</span>
    223. 

  223. </code></pre> <div class="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，如果变量<code>x</code>小于等于<code>0</code>，就手动抛出一个错误，告诉用户<code>x</code>的值不正确，整个程序就会在这里中断执行。可以看到，<code>throw</code>抛出的错误就是它的参数，这里是一个<code>Error</code>实例。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">UserError</span><span class="token punctuation">(</span><span class="token parameter">message</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    224. 

  224.   <span class="token keyword">this</span><span class="token punctuation">.</span>message <span class="token operator">=</span> message <span class="token operator">||</span> <span class="token string">'默认信息'</span><span class="token punctuation">;</span>
    225. 

  225.   <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">'UserError'</span><span class="token punctuation">;</span>
    226. 

  226. <span class="token punctuation">}</span>
    227. 

  227. 

  228. <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">UserError</span><span class="token punctuation">(</span><span class="token string">'出错了！'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    229. 

  229. <span class="token comment">// Uncaught UserError {message: &quot;出错了！&quot;, name: &quot;UserError&quot;}</span>
    230. 

  230. </code></pre> <div class="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>throw</code>抛出的是一个<code>UserError</code>实例。</p> <p>实际上，<code>throw</code>可以抛出任何类型的值。也就是说，它的参数可以是任何值。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 抛出一个字符串</span>
    231. 

  231. <span class="token keyword">throw</span> <span class="token string">'Error！'</span><span class="token punctuation">;</span>
    232. 

  232. <span class="token comment">// Uncaught Error！</span>
    233. 

  233. 

  234. <span class="token comment">// 抛出一个数值</span>
    235. 

  235. <span class="token keyword">throw</span> <span class="token number">42</span><span class="token punctuation">;</span>
    236. 

  236. <span class="token comment">// Uncaught 42</span>
    237. 

  237. 

  238. <span class="token comment">// 抛出一个布尔值</span>
    239. 

  239. <span class="token keyword">throw</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    240. 

  240. <span class="token comment">// Uncaught true</span>
    241. 

  241. 

  242. <span class="token comment">// 抛出一个对象</span>
    243. 

  243. <span class="token keyword">throw</span> <span class="token punctuation">{</span>
    244. 

  244.   <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>
    245. 

  245.     <span class="token keyword">return</span> <span class="token string">'Error!'</span><span class="token punctuation">;</span>
    246. 

  246.   <span class="token punctuation">}</span>
    247. 

  247. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    248. 

  248. <span class="token comment">// Uncaught {toString: ƒ}</span>
    249. 

  249. </code></pre> <div class="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>对于 JavaScript 引擎来说，<strong>遇到<code>throw</code>语句，程序就中止了</strong>。引擎会接收到<code>throw</code>抛出的信息，可能是一个错误实例，也可能是其他类型的值。</p> <h3 id="_5、try-catch-结构-捕获错误-对错误进行处理-不中断"><a href="#_5、try-catch-结构-捕获错误-对错误进行处理-不中断" class="header-anchor">#</a> 5、try...catch 结构（捕获错误，对错误进行处理，不中断）</h3> <p>一旦发生错误，程序就中止执行了。JavaScript 提供了<code>try...catch</code>结构，<strong>允许对错误进行处理</strong>，选择是否往下执行。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">try</span> <span class="token punctuation">{</span>
    250. 

  250.   <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">'出错了!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    251. 

  251. <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>
    252. 

  252.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">&quot;: &quot;</span> <span class="token operator">+</span> e<span class="token punctuation">.</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span>
    253. 

  253.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>stack<span class="token punctuation">)</span><span class="token punctuation">;</span>
    254. 

  254. <span class="token punctuation">}</span>
    255. 

  255. <span class="token comment">// Error: 出错了!</span>
    256. 

  256. <span class="token comment">//   at &lt;anonymous&gt;:3:9</span>
    257. 

  257. <span class="token comment">//   ...</span>
    258. 

  258. </code></pre> <div class="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>try</code>代码块抛出错误（上例用的是<code>throw</code>语句），JavaScript 引擎就立即把代码的执行，转到<code>catch</code>代码块，或者说错误被<code>catch</code>代码块捕获了。<code>catch</code>接受一个参数，表示<code>try</code>代码块抛出的值。</p> <p>如果你不确定某些代码是否会报错，就可以把它们放在<code>try...catch</code>代码块之中，便于进一步对错误进行处理。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">try</span> <span class="token punctuation">{</span>
    259. 

  259.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    260. 

  260. <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>
    261. 

  261.   <span class="token comment">// 处理错误</span>
    262. 

  262. <span class="token punctuation">}</span>
    263. 

  263. </code></pre> <div class="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，如果函数<code>f</code>执行报错，就会进行<code>catch</code>代码块，接着对错误进行处理。</p> <p><code>catch</code>代码块捕获错误之后，<strong>程序不会中断</strong>，会按照正常流程继续执行下去。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">try</span> <span class="token punctuation">{</span>
    264. 

  264.   <span class="token keyword">throw</span> <span class="token string">&quot;出错了&quot;</span><span class="token punctuation">;</span>
    265. 

  265. <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>
    266. 

  266.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">111</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    267. 

  267. <span class="token punctuation">}</span>
    268. 

  268. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">222</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    269. 

  269. <span class="token comment">// 111</span>
    270. 

  270. <span class="token comment">// 222</span>
    271. 

  271. </code></pre> <div class="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>try</code>代码块抛出的错误，被<code>catch</code>代码块捕获后，程序会继续向下执行。</p> <p><code>catch</code>代码块之中，还可以再抛出错误，甚至使用嵌套的<code>try...catch</code>结构。</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">100</span><span class="token punctuation">;</span>
    272. 

  272. 

  273. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    274. 

  274.   <span class="token keyword">throw</span> n<span class="token punctuation">;</span>
    275. 

  275. <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>
    276. 

  276.   <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token operator">&lt;=</span> <span class="token number">50</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    277. 

  277.     <span class="token comment">// ...</span>
    278. 

  278.   <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    279. 

  279.     <span class="token keyword">throw</span> e<span class="token punctuation">;</span>
    280. 

  280.   <span class="token punctuation">}</span>
    281. 

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

  282. <span class="token comment">// Uncaught 100</span>
    283. 

  283. </code></pre> <div class="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>catch</code>代码之中又抛出了一个错误。</p> <p>为了<strong>捕捉不同类型的错误</strong>，<code>catch</code>代码块之中可以加入判断语句。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">try</span> <span class="token punctuation">{</span>
    284. 

  284.   foo<span class="token punctuation">.</span><span class="token function">bar</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    285. 

  285. <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>
    286. 

  286.   <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token keyword">instanceof</span> <span class="token class-name">EvalError</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    287. 

  287.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">&quot;: &quot;</span> <span class="token operator">+</span> e<span class="token punctuation">.</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span>
    288. 

  288.   <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token keyword">instanceof</span> <span class="token class-name">RangeError</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    289. 

  289.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>name <span class="token operator">+</span> <span class="token string">&quot;: &quot;</span> <span class="token operator">+</span> e<span class="token punctuation">.</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span>
    290. 

  290.   <span class="token punctuation">}</span>
    291. 

  291.   <span class="token comment">// ...</span>
    292. 

  292. <span class="token punctuation">}</span>
    293. 

  293. </code></pre> <div class="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>catch</code>捕获错误之后，会判断错误类型（<code>EvalError</code>还是<code>RangeError</code>），进行不同的处理。</p> <h3 id="_6、finally-代码块-在try-catch最后-必执行的"><a href="#_6、finally-代码块-在try-catch最后-必执行的" class="header-anchor">#</a> 6、finally 代码块（在try..catch最后，必执行的）</h3> <p><code>try...catch</code>结构允许在最后添加一个<code>finally</code>代码块，表示<strong>不管是否出现错误，都必需在最后运行的语句</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">cleansUp</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    294. 

  294.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    295. 

  295.     <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">'出错了……'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    296. 

  296.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'此行不会执行'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    297. 

  297.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span> <span class="token comment">// 不管有没有出错都会执行finally</span>
    298. 

  298.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'完成清理工作'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    299. 

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

  300.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'此行不会执行'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    301. 

  301. <span class="token punctuation">}</span>
    302. 

  302. 

  303. <span class="token function">cleansUp</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    304. 

  304. <span class="token comment">// 完成清理工作</span>
    305. 

  305. <span class="token comment">// Uncaught Error: 出错了……</span>
    306. 

  306. <span class="token comment">//    at cleansUp (&lt;anonymous&gt;:3:11)</span>
    307. 

  307. <span class="token comment">//    at &lt;anonymous&gt;:10:1</span>
    308. 

  308. 

  309. <span class="token comment">// 由于没有catch语句，所以会打印出错误信息，并中断除finally以外的代码。</span>
    310. 

  310. <span class="token comment">// 如果有catch语句则会执行catch内的代码块，而不会打印错误信息。且不会中断代码。</span>
    311. 

  311. </code></pre> <div class="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，由于没有<code>catch</code>语句块，一旦发生错误，代码就会中断执行。中断执行之前，会先执行<code>finally</code>代码块，然后再向用户提示报错信息。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">idle</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    312. 

  312.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    313. 

  313.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    314. 

  314.     <span class="token keyword">return</span> <span class="token string">'result'</span><span class="token punctuation">;</span>
    315. 

  315.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    316. 

  316.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'FINALLY'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    317. 

  317.   <span class="token punctuation">}</span>
    318. 

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

  319. 

  320. <span class="token function">idle</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span>
    321. 

  321. <span class="token comment">// hello</span>
    322. 

  322. <span class="token comment">// FINALLY</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><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，<code>try</code>代码块没有发生错误，而且里面还包括<code>return</code>语句，但是<code>finally</code>代码块依然会执行。而且，这个函数的返回值还是<code>result</code>。</p> <p>下面的例子说明，<code>return</code>语句的执行是排在<code>finally</code>代码之前，只是等<code>finally</code>代码执行完毕后才返回。</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>
    324. 

  324. <span class="token keyword">function</span> <span class="token function">countUp</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    325. 

  325.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    326. 

  326.     <span class="token keyword">return</span> count<span class="token punctuation">;</span>
    327. 

  327.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    328. 

  328.     count<span class="token operator">++</span><span class="token punctuation">;</span>
    329. 

  329.   <span class="token punctuation">}</span>
    330. 

  330. <span class="token punctuation">}</span>
    331. 

  331. 

  332. <span class="token function">countUp</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    333. 

  333. <span class="token comment">// 0</span>
    334. 

  334. count
    335. 

  335. <span class="token comment">// 1</span>
    336. 

  336. </code></pre> <div class="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>上面代码说明，<code>return</code>语句里面的<code>count</code>的值，是在<code>finally</code>代码块运行之前就获取了。</p> <p>下面是<code>finally</code>代码块用法的<strong>典型场景</strong>。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">openFile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 打开文件</span>
    337. 

  337. 

  338. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    339. 

  339.   <span class="token function">writeFile</span><span class="token punctuation">(</span>Data<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 写入文件</span>
    340. 

  340. <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>
    341. 

  341.   <span class="token function">handleError</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 如果写入出错这处理错误</span>
    342. 

  342. <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    343. 

  343.   <span class="token function">closeFile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// 不管是否出错都会执行关闭文件</span>
    344. 

  344. <span class="token punctuation">}</span>
    345. 

  345. </code></pre> <div class="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>上面代码首先打开一个文件，然后在<code>try</code>代码块中写入文件，如果没有发生错误，则运行<code>finally</code>代码块关闭文件；一旦发生错误，则先使用<code>catch</code>代码块处理错误，再使用<code>finally</code>代码块关闭文件。</p> <p>下面的例子充分反映了<code>try...catch...finally</code>这三者之间的<strong>执行顺序</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 punctuation">)</span> <span class="token punctuation">{</span>
    346. 

  346.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    347. 

  347.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    348. 

  348.     <span class="token keyword">throw</span> <span class="token string">'bug'</span><span class="token punctuation">;</span>
    349. 

  349.   <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>
    350. 

  350.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    351. 

  351.     <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span> <span class="token comment">// 这句原本会延迟到 finally 代码块结束再执行</span>
    352. 

  352.     console<span class="token punctuation">.</span><span class="token function">log</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">// 不会运行</span>
    353. 

  353.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    354. 

  354.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    355. 

  355.     <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span> <span class="token comment">// 这句会覆盖掉前面那句 return</span>
    356. 

  356.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 不会运行</span>
    357. 

  357.   <span class="token punctuation">}</span>
    358. 

  358. 

  359.   console<span class="token punctuation">.</span><span class="token function">log</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">// 不会运行</span>
    360. 

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

  361. 

  362. <span class="token keyword">var</span> result <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    363. 

  363. <span class="token comment">// 0</span>
    364. 

  364. <span class="token comment">// 1</span>
    365. 

  365. <span class="token comment">// 3</span>
    366. 

  366. 

  367. result
    368. 

  368. <span class="token comment">// false</span>
    369. 

  369. </code></pre> <div class="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，<code>catch</code>代码块结束执行之前，会先执行<code>finally</code>代码块。</p> <p><code>catch</code>代码块之中，触发转入<code>finally</code>代码块的标志，不仅有<code>return</code>语句，还有<code>throw</code>语句。</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 punctuation">)</span> <span class="token punctuation">{</span>
    370. 

  370.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    371. 

  371.     <span class="token keyword">throw</span> <span class="token string">'出错了！'</span><span class="token punctuation">;</span>
    372. 

  372.   <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>
    373. 

  373.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'捕捉到内部错误'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    374. 

  374.     <span class="token keyword">throw</span> e<span class="token punctuation">;</span> <span class="token comment">// 这句原本会等到finally结束再执行</span>
    375. 

  375.   <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    376. 

  376.     <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span> <span class="token comment">// 直接返回</span>
    377. 

  377.   <span class="token punctuation">}</span>
    378. 

  378. <span class="token punctuation">}</span>
    379. 

  379. 

  380. <span class="token keyword">try</span> <span class="token punctuation">{</span>
    381. 

  381.   <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 接收到返回值false，并没有接收到错误</span>
    382. 

  382. <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>
    383. 

  383.   <span class="token comment">// 此处不会执行</span>
    384. 

  384.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'caught outer &quot;bogus&quot;'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    385. 

  385. <span class="token punctuation">}</span>
    386. 

  386. 

  387. <span class="token comment">//  捕捉到内部错误</span>
    388. 

  388. </code></pre> <div class="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>上面代码中，进入<code>catch</code>代码块之后，一遇到<code>throw</code>语句，就会去执行<code>finally</code>代码块，其中有<code>return false</code>语句，因此就直接返回了，不再会回去执行<code>catch</code>代码块剩下的部分了。</p> <p><code>try</code>代码块内部，还可以再使用<code>try</code>代码块。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">try</span> <span class="token punctuation">{</span>
    389. 

  389.   <span class="token keyword">try</span> <span class="token punctuation">{</span>
    390. 

  390.     consle<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Hello world!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 报错，console拼错</span>
    391. 

  391.   <span class="token punctuation">}</span>
    392. 

  392.   <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    393. 

  393.     console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Finally'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    394. 

  394.   <span class="token punctuation">}</span>
    395. 

  395.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Will I run?'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    396. 

  396. <span class="token punctuation">}</span> <span class="token keyword">catch</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    397. 

  397.   console<span class="token punctuation">.</span><span class="token function">error</span><span class="token punctuation">(</span>error<span class="token punctuation">.</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span>
    398. 

  398. <span class="token punctuation">}</span>
    399. 

  399. <span class="token comment">// Finally</span>
    400. 

  400. <span class="token comment">// consle is not defined</span>
    401. 

  401. </code></pre> <div class="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>try</code>里面还有一个<code>try</code>。内层的<code>try</code>报错（<code>console</code>拼错了），这时会执行内层的<code>finally</code>代码块，然后抛出错误，被外层的<code>catch</code>捕获。</p> <h2 id="四、console对象"><a href="#四、console对象" class="header-anchor">#</a> 四、console对象</h2> <h3 id="console-time-console-timeend-用于计算程序的用时"><a href="#console-time-console-timeend-用于计算程序的用时" class="header-anchor">#</a> console.time()，console.timeEnd() （用于计算程序的用时）</h3> <p>这两个方法用于计时，可以算出一个操作所花费的准确时间。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>console.time('Array initialize');
    402. 

  402. 

  403. var array= new Array(1000000);
    404. 

  404. for (var i = array.length - 1; i &gt;= 0; i--) {
    405. 

  405.   array[i] = new Object();
    406. 

  406. };
    407. 

  407. 

  408. console.timeEnd('Array initialize');
    409. 

  409. // Array initialize: 1914.481ms
    410. 

  410. </code></pre> <div class="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>time</code>方法表示计时开始，<code>timeEnd</code>方法表示计时结束。它们的参数是计时器的名称。调用<code>timeEnd</code>方法之后，控制台会显示“计时器名称: 所耗费的时间”。</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/01.基础.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/74d2ab3fbfeaaa68/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">内置对象</div></a></div> <div class="page-nav"><p class="inner"><!----> <span class="next"><a href="/blog/pages/74d2ab3fbfeaaa68/">内置对象</a>→
    411. 

  411.       </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>
    412. 

  412.             Git修改分支名
    413. 

  413.             <!----></div></a> <span class="date">08-11</span></dt></dl><dl><dd>02</dd> <dt><a href="/blog/pages/55f894/"><div>
    414. 

  414.             CSS给table的tbody添加滚动条
    415. 

  415.             <!----></div></a> <span class="date">06-29</span></dt></dl><dl><dd>03</dd> <dt><a href="/blog/pages/829589/"><div>
    416. 

  416.             我做了一个手写春联小网页，祝大家虎年暴富
    417. 

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    418. 

  418.               原创
    419. 

  419.             </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> 
    420. 

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    421. 

  421.     <span>Hesb | <a href="https://github.com/heBody/blob" target="_blank">MIT License</a></span></div> <div class="buttons"><div title="返回顶部" class="button blur go-to-top iconfont icon-fanhuidingbu" style="display:none;"></div> <div title="去评论" class="button blur go-to-comment iconfont icon-pinglun" style="display:none;"></div> <div title="主题模式" class="button blur theme-mode-but iconfont icon-zhuti"><ul class="select-box" style="display:none;"><li class="iconfont icon-zidong">
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