(window.webpackJsonp=window.webpackJsonp||[]).push([[138],{467: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:"面向对象编程"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#面向对象编程"}},[s._v("#")]),s._v(" 面向对象编程")]),s._v(" "),t("h2",{attrs:{id:"一、实例对象与new命令"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#一、实例对象与new命令"}},[s._v("#")]),s._v(" 一、实例对象与new命令")]),s._v(" "),t("h3",{attrs:{id:"_1、对象是什么"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_1、对象是什么"}},[s._v("#")]),s._v(" 1、对象是什么")]),s._v(" "),t("p",[s._v("面向对象编程（Object Oriented Programming，缩写为 OOP）是目前主流的编程范式。它将真实世界各种复杂的关系，抽象为一个个对象，然后由对象之间的分工与合作，完成对真实世界的模拟。")]),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("p",[t("code",[s._v("new")]),s._v("命令本身就可以执行构造函数，所以后面的构造函数可以带括号，也"),t("strong",[s._v("可以不带括号")]),s._v("。下面两行代码是等价的，但是为了表示这里是函数调用，"),t("strong",[s._v("推荐使用括号")]),s._v("。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 推荐的写法")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" v "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),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("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("上面代码的"),t("code",[s._v("Fubar")]),s._v("为构造函数，"),t("code",[s._v("use strict")]),s._v("命令保证了该函数在严格模式下运行。由于"),t("strong",[s._v("严格模式中，函数内部的"),t("code",[s._v("this")]),s._v("不能指向全局对象")]),s._v("，默认等于"),t("code",[s._v("undefined")]),s._v("，导致不加"),t("code",[s._v("new")]),s._v("调用会报错（JavaScript 不允许对"),t("code",[s._v("undefined")]),s._v("添加属性）。")]),s._v(" "),t("p",[t("strong",[s._v("另一个解决办法")]),s._v("，构造函数内部判断是否使用"),t("code",[s._v("new")]),s._v("命令，如果发现没有使用，则直接返回一个实例对象。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("Fubar")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("foo"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" bar")]),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 comment"}},[s._v("// instanceof运算符用于检测构造函数的prototype属性是否出现在某个实例对象的原型链（），返回布尔值。")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 语法： <实例对象> instanceof <构造函数>")]),s._v("\n  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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("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br")])]),t("p",[s._v("上面代码中的构造函数，不管加不加"),t("code",[s._v("new")]),s._v("命令，都会得到同样的结果。")]),s._v(" "),t("h4",{attrs:{id:"new-命令的原理"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#new-命令的原理"}},[s._v("#")]),s._v(" new 命令的原理")]),s._v(" "),t("p",[s._v("使用"),t("code",[s._v("new")]),s._v("命令时，它后面的函数依次执行下面的步骤。")]),s._v(" "),t("ol",[t("li",[t("p",[s._v("创建一个空对象，作为将要返回的实例对象。")])]),s._v(" "),t("li",[t("p",[s._v("将这个空对象的原型，指向构造函数的"),t("code",[s._v("prototype")]),s._v("属性。")])]),s._v(" "),t("li",[t("p",[s._v("将这个空对象赋值给函数内部的"),t("code",[s._v("this")]),s._v("关键字。")])]),s._v(" "),t("li",[t("p",[s._v("开始执行构造函数内部的代码。（代码中this指向空对象（实例对象））")])]),s._v(" "),t("li",[t("p",[s._v("返回实例对象（或自定义对象）")])])]),s._v(" "),t("p",[s._v("也就是说，构造函数内部，"),t("code",[s._v("this")]),s._v("指的是一个新生成的空对象，"),t("strong",[s._v("所有针对"),t("code",[s._v("this")]),s._v("的操作，都会发生在这个空对象上")]),s._v("。构造函数之所以叫“构造函数”，就是说这个函数的目的，就是操作一个空对象（即"),t("code",[s._v("this")]),s._v("对象），将其“构造”为需要的样子。")]),s._v(" "),t("p",[s._v("如果构造函数内部有"),t("code",[s._v("return")]),s._v("语句，而且"),t("code",[s._v("return")]),s._v("后面跟着一个对象，"),t("code",[s._v("new")]),s._v("命令会返回"),t("code",[s._v("return")]),s._v("语句指定的对象；否则，就会不管"),t("code",[s._v("return")]),s._v("语句，返回"),t("code",[s._v("this")]),s._v("对象。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function-variable function"}},[s._v("Vehicle")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),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 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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("p",[s._v("上面代码中，构造函数"),t("code",[s._v("Vehicle")]),s._v("的"),t("code",[s._v("return")]),s._v("语句返回一个数值。这时，"),t("code",[s._v("new")]),s._v("命令就会忽略这个"),t("code",[s._v("return")]),s._v("语句，返回“构造”后的"),t("code",[s._v("this")]),s._v("对象。")]),s._v(" "),t("p",[s._v("但是，如果"),t("code",[s._v("return")]),s._v("语句返回的是一个跟"),t("code",[s._v("this")]),s._v("无关的新对象，"),t("code",[s._v("new")]),s._v("命令会返回这个新对象，而不是"),t("code",[s._v("this")]),s._v("对象。这一点需要特别引起注意。")]),s._v(" 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"),t("h4",{attrs:{id:"_1-构造函数的缺点"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_1-构造函数的缺点"}},[s._v("#")]),s._v(" （1）构造函数的缺点")]),s._v(" "),t("p",[s._v("JavaScript 通过构造函数生成新对象，因此"),t("strong",[s._v("构造函数可以视为对象的模板")]),s._v("。实例对象的属性和方法，可以定义在构造函数内部。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("Cat")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("name"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" color")]),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 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"),t("p",[t("strong",[s._v("这个问题的解决方法，就是 JavaScript 的原型对象（prototype）。")])]),s._v(" "),t("h4",{attrs:{id:"_2-prototype-属性的作用"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_2-prototype-属性的作用"}},[s._v("#")]),s._v(" （2）prototype 属性的作用")]),s._v(" "),t("p",[t("strong",[s._v("JavaScript 继承机制的设计思想就是，原型对象的所有属性和方法，都能被实例对象共享")]),s._v("。也就是说，如果属性和方法定义在原型上，那么所有实例对象就能共享，不仅节省了内存，还体现了实例对象之间的联系。")]),s._v(" "),t("p",[s._v("下面，先看怎么为对象指定原型。JavaScript 规定，"),t("strong",[s._v("每个函数都有一个"),t("code",[s._v("prototype")]),s._v("属性，指向一个对象")]),s._v("。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),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 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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("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br"),t("span",{staticClass:"line-number"},[s._v("14")]),t("br"),t("span",{staticClass:"line-number"},[s._v("15")]),t("br"),t("span",{staticClass:"line-number"},[s._v("16")]),t("br"),t("span",{staticClass:"line-number"},[s._v("17")]),t("br"),t("span",{staticClass:"line-number"},[s._v("18")]),t("br")])]),t("p",[s._v("另一种更简单的写法，是利用 ES2017 才引入标准的"),t("code",[s._v("Object.getOwnPropertyDescriptors")]),s._v("方法。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("copyObject")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("orig")]),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("return")]),s._v(" 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("\n    Object"),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("h2",{attrs:{id:"五、严格模式"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#五、严格模式"}},[s._v("#")]),s._v(" 五、严格模式")]),s._v(" "),t("p",[s._v("除了正常的运行模式，JavaScript 还有第二种运行模式：严格模式（strict mode）。顾名思义，这种模式采用更加严格的 JavaScript 语法。")]),s._v(" "),t("p",[s._v("同样的代码，在正常模式和严格模式中，可能会有不一样的运行结果。一些在正常模式下可以运行的语句，在严格模式下将不能运行。")]),s._v(" "),t("h3",{attrs:{id:"_1、设计目的"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_1、设计目的"}},[s._v("#")]),s._v(" 1、设计目的")]),s._v(" "),t("p",[s._v("早期的 JavaScript 语言有很多设计不合理的地方，但是为了兼容以前的代码，又不能改变老的语法，只能不断添加新的语法，引导程序员使用新语法。")]),s._v(" "),t("p",[s._v("严格模式是从 ES5 进入标准的，主要目的有以下几个。")]),s._v(" "),t("ul",[t("li",[s._v("明确禁止一些不合理、不严谨的语法，减少 JavaScript 语言的一些怪异行为。")]),s._v(" "),t("li",[s._v("增加更多报错的场合，消除代码运行的一些不安全之处，保证代码运行的安全。")]),s._v(" "),t("li",[s._v("提高编译器效率，增加运行速度。")]),s._v(" "),t("li",[s._v("为未来新版本的 JavaScript 语法做好铺垫。")])]),s._v(" "),t("p",[s._v("总之，严格模式体现了 JavaScript 更合理、更安全、更严谨的发展方向。")]),s._v(" "),t("h3",{attrs:{id:"_2、启用方法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_2、启用方法"}},[s._v("#")]),s._v(" 2、启用方法")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'use strict'")]),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("blockquote",[t("p",[s._v("更多关于严格模式内容："),t("a",{attrs:{href:"https://wangdoc.com/javascript/oop/strict.html",target:"_blank",rel:"noopener noreferrer"}},[s._v("https://wangdoc.com/javascript/oop/strict.html"),t("OutboundLink")],1)])]),s._v(" "),t("h2",{attrs:{id:"文档"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#文档"}},[s._v("#")]),s._v(" 文档")]),s._v(" "),t("p",[s._v("学习文档："),t("a",{attrs:{href:"https://wangdoc.com/javascript/",target:"_blank",rel:"noopener noreferrer"}},[s._v("https://wangdoc.com/javascript/"),t("OutboundLink")],1)])])}),[],!1,null,null,null);t.default=e.exports}}]);