(window.webpackJsonp=window.webpackJsonp||[]).push([[97],{427:function(s,t,a){"use strict";a.r(t);var n=a(3),e=Object(n.a)({},(function(){var s=this,t=s._self._c;return t("ContentSlotsDistributor",{attrs:{"slot-key":s.$parent.slotKey}},[t("h1",{attrs:{id:"对象的扩展"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#对象的扩展"}},[s._v("#")]),s._v(" 对象的扩展")]),s._v(" "),t("p",[s._v("对象（object）是 JavaScript 最重要的数据结构。ES6 对它进行了重大升级，本章介绍数据结构本身的改变，下一章介绍"),t("code",[s._v("Object")]),s._v("对象的新增方法。\n")]),s._v(" "),t("h2",{attrs:{id:"属性的简洁表示法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#属性的简洁表示法"}},[s._v("#")]),s._v(" 属性的简洁表示法")]),s._v(" "),t("p",[s._v("ES6 允许在大括号里面，"),t("strong",[s._v("直接写入变量和函数，作为对象的属性和方法")]),s._v("。这样的书写更加简洁。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" foo "),t("span",{pre:!0,attrs:{class:"token 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"),t("p",[s._v("注意，"),t("strong",[s._v("简写的对象方法不能用作构造函数")]),s._v("，会报错。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" obj "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("this")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("foo "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br")])]),t("p",[s._v("上面代码的方法一是直接用标识符作为属性名，方法二是用表达式作为属性名，这时要将表达式放在方括号之内。")]),s._v(" "),t("p",[s._v("但是，如果使用字面量方式定义对象（使用大括号），在 ES5 中只能使用方法一（标识符）定义属性。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" obj "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("p",[s._v("下面是另一个例子。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" lastWord "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'last word'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" a "),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("表达式还可以用于定义方法名。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" obj "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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number"}},[s._v(".1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// {}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("上面代码中，扩展运算符后面是整数"),t("code",[s._v("1")]),s._v("，会自动转为数值的包装对象"),t("code",[s._v("Number{1}")]),s._v("。由于该对象没有自身属性，所以返回一个空对象。")]),s._v(" "),t("p",[s._v("下面的例子都是类似的道理。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 等同于 {...Object(true)}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("true")]),t("span",{pre:!0,attrs:{class:"token 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{}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br")])]),t("p",[s._v("但是，如果扩展运算符后面是字符串，它会自动转成一个类似数组的对象，因此返回的不是空对象。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'hello'")]),t("span",{pre:!0,attrs:{class:"token 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Object"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("create")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("Object"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getPrototypeOf")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("obj"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n  obj\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 写法三")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" clone3 "),t("span",{pre:!0,attrs:{class:"token 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"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" response"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("settings"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("showSplashScreen "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("||")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("true")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br")])]),t("p",[s._v("上面的三行代码都通过"),t("code",[s._v("||")]),s._v("运算符指定默认值，但是这样写是错的。开发者的原意是，只要属性的值为"),t("code",[s._v("null")]),s._v("或"),t("code",[s._v("undefined")]),s._v("，默认值就会生效，但是属性的值如果为空字符串或"),t("code",[s._v("false")]),s._v("或"),t("code",[s._v("0")]),s._v("，默认值也会生效。")]),s._v(" "),t("p",[s._v("为了避免这种情况，"),t("a",{attrs:{href:"https://github.com/tc39/proposal-nullish-coalescing",target:"_blank",rel:"noopener noreferrer"}},[s._v("ES2020"),t("OutboundLink")],1),s._v(" 引入了一个新的 Null 判断运算符"),t("code",[s._v("??")]),s._v("。它的行为类似"),t("code",[s._v("||")]),s._v("，但是只有运算符左侧的值为"),t("code",[s._v("null")]),s._v("或"),t("code",[s._v("undefined")]),s._v("时，才会返回右侧的值。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" headerText "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" response"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("settings"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("headerText "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("??")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Hello, 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("p",[t("code",[s._v("??")]),s._v("有一个运算优先级问题，它与"),t("code",[s._v("&&")]),s._v("和"),t("code",[s._v("||")]),s._v("的优先级孰高孰低。现在的规则是，如果多个逻辑运算符一起使用，必须用括号表明优先级，否则会报错。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 报错")]),s._v("\nlhs "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("&&")]),s._v(" middle "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("??")]),s._v(" rhs\nlhs "),t("span",{pre:!0,attrs:{class:"token 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