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href="/article/%E4%B8%80%E6%96%87%E8%AF%BB%E6%87%82%E4%BC%AA%E7%B1%BB%E4%B8%8E%E4%BC%AA%E5%85%83%E7%B4%A0.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>一文读懂伪类与伪元素</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E4%B8%80%E6%96%87%E8%AF%BB%E6%87%82%E5%87%BD%E6%95%B0%E4%B8%ADthis%E6%8C%87%E5%90%91%E9%97%AE%E9%A2%98.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>一文读懂函数中this指向问题</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E4%BB%8E0%E5%AE%9E%E7%8E%B0%E4%B8%80%E4%B8%AA%E5%B9%B4%E5%BA%A6%E6%8A%A5%E5%91%8A.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>从0实现一个年度报告</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E5%BD%BB%E5%BA%95%E6%90%9E%E6%87%82%E5%AF%B9%E8%B1%A1%E7%9A%84%E6%95%B0%E6%8D%AE%E5%B1%9E%E6%80%A7%E6%8F%8F%E8%BF%B0%E7%AC%A6%E3%80%81%E5%AD%98%E5%82%A8%E5%B1%9E%E6%80%A7%E6%8F%8F%E8%BF%B0%E7%AC%A6.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>彻底搞懂对象的数据属性描述符、存储属性描述符</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E6%B5%85%E6%9E%90defineProperty%E4%B8%8EProxy%E5%AE%9E%E7%8E%B0%E7%9A%84%E5%8F%8C%E5%90%91%E7%BB%91%E5%AE%9A.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>浅析defineProperty与Proxy实现的双向绑定</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5JavaScript%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>深入JavaScript数据类型</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5Vue3%E6%BA%90%E7%A0%81%EF%BC%8C%E7%9C%8B%E7%9C%8BVue.use%E5%90%8E%E7%A9%B6%E7%AB%9F%E5%8F%91%E7%94%9F%E4%BA%86%E4%BB%80%E4%B9%88%EF%BC%9F.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>深入Vue3源码，看看Vue.use后究竟发生了什么？</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3Proxy%E4%B8%8EReflect.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>深入理解Proxy与Reflect</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3%E6%B5%8F%E8%A7%88%E5%99%A8%E7%BC%93%E5%AD%98%E6%9C%BA%E5%88%B6.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>深入理解浏览器缓存机制</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/article/%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3%E6%B5%8F%E8%A7%88%E5%99%A8%E8%BF%90%E8%A1%8C%E5%8E%9F%E7%90%86.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>深入理解浏览器运行原理</p><!--]--></a><!----></div><!----></div><!--]--></div></section></div><div class="group" data-v-7e25b47d><section class="VPSidebarItem level-0 collapsible collapsed" data-v-7e25b47d data-v-f0fc17b5><div class="item" role="button" tabindex="0" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><h2 class="text" data-v-f0fc17b5>学习笔记</h2><div class="caret" role="button" aria-label="toggle section" tabindex="0" data-v-f0fc17b5><span class="vpi-chevron-right caret-icon" data-v-f0fc17b5></span></div></div><div class="items" data-v-f0fc17b5><!--[--><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/JavaScript.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>JavaScript基础</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/CSS.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>CSS基础</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/JavaScriptEnhanced.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>JavaScript进阶</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/Front-end%20Engineering.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>前端工程化</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/SSR.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>服务端渲染</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/React.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>React基础</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/React%20Hooks.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>React Hooks</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/Redux.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>Redux</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/React%20Router.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>React Router</p><!--]--></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-f0fc17b5 data-v-f0fc17b5><div class="item" data-v-f0fc17b5><div class="indicator" data-v-f0fc17b5></div><a class="VPLink link link" href="/note/MySQL.html" data-v-f0fc17b5><!--[--><p class="text" data-v-f0fc17b5>MySQL</p><!--]--></a><!----></div><!----></div><!--]--></div></section></div><!--]--><!--[--><!--]--></nav></aside><div class="VPContent has-sidebar" id="VPContent" data-v-f671d7ba data-v-47a44795><div class="VPDoc has-sidebar has-aside" data-v-47a44795 data-v-e584b61a><!--[--><!--]--><div class="container" data-v-e584b61a><div class="aside" data-v-e584b61a><div class="aside-curtain" data-v-e584b61a></div><div class="aside-container" data-v-e584b61a><div class="aside-content" data-v-e584b61a><div class="VPDocAside" data-v-e584b61a data-v-f65316ad><!--[--><!--]--><!--[--><!--]--><div class="VPDocAsideOutline" role="navigation" data-v-f65316ad data-v-c9c7be2d><div class="content" data-v-c9c7be2d><div class="outline-marker" data-v-c9c7be2d></div><div class="outline-title" role="heading" aria-level="2" data-v-c9c7be2d>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-c9c7be2d><span class="visually-hidden" id="doc-outline-aria-label" data-v-c9c7be2d> Table of Contents for current page </span><ul class="VPDocOutlineItem root" data-v-c9c7be2d data-v-ec8a1b48><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-f65316ad></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-e584b61a><div class="content-container" data-v-e584b61a><!--[--><!--]--><main class="main" data-v-e584b61a><div style="position:relative;" class="vp-doc _article_%E4%B8%80%E6%96%87%E8%AF%BB%E6%87%82%E5%87%BD%E6%95%B0%E4%B8%ADthis%E6%8C%87%E5%90%91%E9%97%AE%E9%A2%98" data-v-e584b61a><div><h1 id="一文读懂函数中this指向问题" tabindex="-1">一文读懂函数中this指向问题 <a class="header-anchor" href="#一文读懂函数中this指向问题" aria-label="Permalink to "一文读懂函数中this指向问题""></a></h1><h2 id="函数中this指向" tabindex="-1">函数中this指向 <a class="header-anchor" href="#函数中this指向" aria-label="Permalink to "函数中this指向""></a></h2><p>函数在调用时, Javascript会默认为this绑定一个值</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>// 定义一个函数</span></span>
|
||
<span class="line"><span>function foo() {</span></span>
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<span class="line"><span> console.log(this)</span></span>
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<span class="line"><span>}</span></span>
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<span class="line"><span></span></span>
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<span class="line"><span>// 1. 直接调用</span></span>
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<span class="line"><span>foo() // Window</span></span>
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<span class="line"><span></span></span>
|
||
<span class="line"><span>// 2. 绑定对象调用</span></span>
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<span class="line"><span>const obj = { name: 'ziu', aaa: foo }</span></span>
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<span class="line"><span>obj.aaa() // obj</span></span>
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<span class="line"><span></span></span>
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<span class="line"><span>// 3. 通过call/apply调用</span></span>
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<span class="line"><span>foo.call('Ziu') // String {'Ziu'}</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>this的绑定:</p><ul><li>和定义的位置没有关系</li><li>和调用方式/调用位置有关系</li><li>是在运行时被绑定的</li></ul><p><strong>this始终指向最后调用它的对象</strong></p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>function foo() {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span>foo() // Window</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>const obj = {</span></span>
|
||
<span class="line"><span> name: 'ziu',</span></span>
|
||
<span class="line"><span> bar: function () {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span>obj.bar() // obj</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>const baz = obj.bar</span></span>
|
||
<span class="line"><span>baz() // Window</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><h2 id="如何改变this的指向" tabindex="-1">如何改变this的指向 <a class="header-anchor" href="#如何改变this的指向" aria-label="Permalink to "如何改变this的指向""></a></h2><h3 id="new-实例化一个函数" tabindex="-1">new 实例化一个函数 <a class="header-anchor" href="#new-实例化一个函数" aria-label="Permalink to "new 实例化一个函数""></a></h3><blockquote><p>new一个对象时发生了什么:</p><ol start="0"><li>创建一个空对象</li><li>这个空对象会被执行prototype连接</li><li>将this指向这个空对象</li><li>执行函数体中的代码</li><li>没有显式返回这个对象时 会默认返回这个对象</li></ol></blockquote><p>函数可以作为一个构造函数, 作为一个类, 可以通过new关键字将其实例化</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>function foo() {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span> this.name = 'Ziu'</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span>foo() // 直接调用的话 this为Window</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>new foo() // 通过new关键字调用 则this指向空对象</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><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="使用-call-apply-bind" tabindex="-1">使用 call apply bind <a class="header-anchor" href="#使用-call-apply-bind" aria-label="Permalink to "使用 call apply bind""></a></h3><p>在 JavaScript 中, 函数是对象。</p><p>JavaScript 函数有它的属性和方法。call() 和 apply() 是预定义的函数方法。</p><p>两个方法可用于调用函数，两个方法的第一个参数必须是对象本身</p><hr><p>要将<code>foo</code>函数中的<code>this</code>指向<code>obj</code>，可以通过赋值的方式：</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>obj.foo = foo // 绑定</span></span>
|
||
<span class="line"><span>obj.foo() // 调用</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>但是也可以通过对函数调用call / apply实现</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>var obj = {</span></span>
|
||
<span class="line"><span> name: 'Ziu'</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>function foo() {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>foo.call(obj) // 将foo执行时的this显式绑定到了obj上 并调用foo</span></span>
|
||
<span class="line"><span>foo.call(123) // foo的this被绑定到了 Number { 123 } 上</span></span>
|
||
<span class="line"><span>foo.call("ziu") // 绑定到了 String { "ziu" } 上</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><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><h4 id="包装类对象" tabindex="-1">包装类对象 <a class="header-anchor" href="#包装类对象" aria-label="Permalink to "包装类对象""></a></h4><p>当我们直接使用类似：</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>"ZiuChen".length // String.length</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br></div></div><p>的语句时，<code>JS</code>会为字符串 <code>ZiuChen</code> 包装一个对象，随后在这个对象上调用 <code>.length</code> 属性</p><h4 id="call和apply的区别" tabindex="-1">call和apply的区别 <a class="header-anchor" href="#call和apply的区别" aria-label="Permalink to "call和apply的区别""></a></h4><ul><li><p>相同点：第一个参数都是相同的，要求传入一个对象</p><ul><li>在函数调用时，会将this绑定到这个传入的对象上</li></ul></li><li><p>不同点：后面的参数</p><ul><li>apply 传入的是一个数组</li><li>call 传入的是参数列表</li></ul></li></ul><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>function foo(name, age, height) {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>foo('Ziu', 18, 1.88)</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>foo.apply('targetThis', 'Ziu', 18, 1.88)</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>foo.call('targetThis', ['Ziu', 18, 1.88])</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><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>当我们需要给一个带参数的函数通过call/apply的方式绑定this时，就需要使用到call/apply的第二个入参了。</p><h4 id="参数列表" tabindex="-1">参数列表 <a class="header-anchor" href="#参数列表" aria-label="Permalink to "参数列表""></a></h4><p>当传入函数的参数有多个时，可以通过<code>...args</code>将参数合并到一个数组中去</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>function foo(...args) {</span></span>
|
||
<span class="line"><span> console.log(args)</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>foo("Ziu", 18, 1.88) // ["Ziu", 18, 1.88]</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><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="bind绑定" tabindex="-1">bind绑定 <a class="header-anchor" href="#bind绑定" aria-label="Permalink to "bind绑定""></a></h4><p>如果我们希望：在每次调用<code>foo</code>时，都能将<code>obj</code>绑定到<code>foo</code>的<code>this</code>上，那么就需要用到<code>bind</code></p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>// 将obj绑定到foo上</span></span>
|
||
<span class="line"><span>const fun = foo.bind(obj)</span></span>
|
||
<span class="line"><span>// 在后续每次调用foo时, foo内的this都将指向obj</span></span>
|
||
<span class="line"><span>fun() // obj</span></span>
|
||
<span class="line"><span>fun() // obj</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><code>bind()</code>方法将创建一个新的函数，当被调用时，将其<code>this</code>关键字</p><h2 id="箭头函数" tabindex="-1">箭头函数 <a class="header-anchor" href="#箭头函数" aria-label="Permalink to "箭头函数""></a></h2><p>箭头函数是<code>ES6</code>新增的编写函数的方式，更简洁。</p><ul><li>箭头函数不会绑定<code>this</code>、<code>argument</code>属性</li><li>箭头函数不能作为构造函数来使用（不能与<code>new</code>同用，会报错）</li></ul><h3 id="箭头函数中的this" tabindex="-1">箭头函数中的this <a class="header-anchor" href="#箭头函数中的this" aria-label="Permalink to "箭头函数中的this""></a></h3><p>在箭头函数中是没有<code>this</code>的：</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>const foo = () => {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span>foo() // window</span></span>
|
||
<span class="line"><span>console.log(this) // window</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><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>之所以找到了<code>Window</code>对象，是因为在调用<code>foo()</code>时，函数内部作用域并没有找到<code>this</code>，转而向上层作用域找<code>this</code></p><p>因此找到了顶层的全局<code>this</code>，也即<code>Window</code>对象</p><h3 id="箭头函数中this的查找规则" tabindex="-1">箭头函数中this的查找规则 <a class="header-anchor" href="#箭头函数中this的查找规则" aria-label="Permalink to "箭头函数中this的查找规则""></a></h3><p>检查以下代码：</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>const obj = {</span></span>
|
||
<span class="line"><span> name: "obj",</span></span>
|
||
<span class="line"><span> foo: function () {</span></span>
|
||
<span class="line"><span> const bar = () => {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span> return bar</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span>const fn = obj.foo()</span></span>
|
||
<span class="line"><span>fn() // obj</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>代码执行完毕，控制台输出<code>this</code>值为<code>obj</code>对象，这是为什么？</p><p>箭头函数中没有<code>this</code>，故会向上层作用域寻找<code>this</code>，<code>bar</code>的上层作用域为函数<code>foo</code>，而函数<code>foo</code>的<code>this</code>由其调用决定</p><p>调用<code>foo</code>函数的为<code>obj</code>对象，故内部箭头函数中的<code>this</code>指向的是<code>obj</code></p><p>检查以下代码：</p><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>const obj = {</span></span>
|
||
<span class="line"><span> name: "obj",</span></span>
|
||
<span class="line"><span> foo: () => {</span></span>
|
||
<span class="line"><span> const bar = () => {</span></span>
|
||
<span class="line"><span> console.log(this)</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span> return bar</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span>const fn = obj.foo()</span></span>
|
||
<span class="line"><span>fn() // Window</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>和上面的代码不同之处在于：<code>foo</code>也是由箭头函数定义的，<code>bar</code>向上找不到<code>foo</code>的<code>this</code>，故而继续向上，找到了全局<code>this</code>，也即<code>Window</code>对象</p><h3 id="严格模式" tabindex="-1">严格模式 <a class="header-anchor" href="#严格模式" aria-label="Permalink to "严格模式""></a></h3><ul><li>在严格模式下，全局的<code>this</code>不是<code>Window</code>对象，而是<code>undefined</code>。</li><li>在 JavaScript 严格模式(strict mode)下, 在调用函数时第一个参数会成为 this 的值， 即使该参数不是一个对象。</li><li>在 JavaScript 非严格模式(non-strict mode)下, 如果第一个参数的值是 null 或 undefined, 它将使用全局对象替代。</li></ul><h2 id="this面试题" tabindex="-1">this面试题 <a class="header-anchor" href="#this面试题" aria-label="Permalink to "this面试题""></a></h2><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>var name = 'window'</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>var person = {</span></span>
|
||
<span class="line"><span> name: 'person',</span></span>
|
||
<span class="line"><span> sayName: function () {</span></span>
|
||
<span class="line"><span> console.log(this.name)</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>function sayName() {</span></span>
|
||
<span class="line"><span> var sss = person.sayName</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span> sss() // 默认绑定: window</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span> person.sayName(); // 隐式绑定: person</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span> (person.sayName)() // 隐式绑定: person, 本质与上一行代码相同</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span> ;(person.sayName = person.sayName)() // 间接调用: window</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>sayName()</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><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></div></div><div class="language- vp-adaptive-theme line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki shiki-themes github-light github-dark vp-code"><code><span class="line"><span>var name = 'window'</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>var person1 = {</span></span>
|
||
<span class="line"><span> name: 'person1',</span></span>
|
||
<span class="line"><span> foo1: function () {</span></span>
|
||
<span class="line"><span> console.log(this.name)</span></span>
|
||
<span class="line"><span> },</span></span>
|
||
<span class="line"><span> foo2: () => console.log(this.name),</span></span>
|
||
<span class="line"><span> foo3: function () {</span></span>
|
||
<span class="line"><span> return function () {</span></span>
|
||
<span class="line"><span> console.log(this.name)</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span> },</span></span>
|
||
<span class="line"><span> foo4: function () {</span></span>
|
||
<span class="line"><span> return () => console.log(this.name)</span></span>
|
||
<span class="line"><span> }</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>var person2 = {</span></span>
|
||
<span class="line"><span> name: 'person2'</span></span>
|
||
<span class="line"><span>}</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>person1.foo1() // 隐式绑定: person1</span></span>
|
||
<span class="line"><span>person1.foo1.call(person2) // 显式绑定: person2</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>person1.foo2() // 上层作用域: window</span></span>
|
||
<span class="line"><span>person1.foo2.call(person2) // 上层作用域: window</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>person1.foo3()() // 默认绑定: window</span></span>
|
||
<span class="line"><span>person1.foo3.call(person2)() // 默认绑定: window</span></span>
|
||
<span class="line"><span>person1.foo3().call(person2) // 显式绑定: person2</span></span>
|
||
<span class="line"><span></span></span>
|
||
<span class="line"><span>person1.foo4()() // 隐式绑定: person1</span></span>
|
||
<span class="line"><span>person1.foo4.call(person2)() // 显式绑定: person2</span></span>
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<span class="line"><span>person1.foo4().call(person2) // 隐式绑定: person1</span></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br></div></div></div></div></main><footer class="VPDocFooter" data-v-e584b61a data-v-80ae8fba><!--[--><!--]--><div class="edit-info" data-v-80ae8fba><div class="edit-link" data-v-80ae8fba><a class="VPLink link vp-external-link-icon no-icon edit-link-button" href="https://github.com/ZiuChen/ZiuChen.github.io/edit/main/docs/article/一文读懂函数中this指向问题.md" target="_blank" rel="noreferrer" data-v-80ae8fba><!--[--><span class="vpi-square-pen edit-link-icon" data-v-80ae8fba></span> Edit this page on GitHub<!--]--></a></div><div class="last-updated" data-v-80ae8fba><p class="VPLastUpdated" data-v-80ae8fba data-v-7aa60d90>Updated Date: <time datetime="2024-04-02T06:24:17.000Z" data-v-7aa60d90></time></p></div></div><nav class="prev-next" data-v-80ae8fba><div class="pager" data-v-80ae8fba><a class="VPLink link pager-link prev" href="/article/%E4%B8%80%E6%96%87%E8%AF%BB%E6%87%82%E4%BC%AA%E7%B1%BB%E4%B8%8E%E4%BC%AA%E5%85%83%E7%B4%A0.html" data-v-80ae8fba><!--[--><span class="desc" data-v-80ae8fba>Previous page</span><span class="title" data-v-80ae8fba>一文读懂伪类与伪元素</span><!--]--></a></div><div class="pager" data-v-80ae8fba><a class="VPLink link pager-link next" href="/article/%E4%BB%8E0%E5%AE%9E%E7%8E%B0%E4%B8%80%E4%B8%AA%E5%B9%B4%E5%BA%A6%E6%8A%A5%E5%91%8A.html" data-v-80ae8fba><!--[--><span class="desc" data-v-80ae8fba>Next page</span><span class="title" data-v-80ae8fba>从0实现一个年度报告</span><!--]--></a></div></nav></footer><!--[--><!--]--></div></div></div><!--[--><!--]--></div></div><footer class="VPFooter has-sidebar" data-v-f671d7ba data-v-1b1e2c98><div class="container" data-v-1b1e2c98><p class="message" data-v-1b1e2c98>Released under the MIT License.</p><p class="copyright" data-v-1b1e2c98>Copyright © 2022-PRESENT ZiuChen</p></div></footer><!--[--><!--]--></div></div>
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