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data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>彻底搞懂对象的数据属性描述符、存储属性描述符</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>浅析defineProperty与Proxy实现的双向绑定</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>深入JavaScript数据类型</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>深入Vue3源码，看看Vue.use后究竟发生了什么？</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>深入理解Proxy与Reflect</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>深入理解浏览器缓存机制</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></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-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>深入理解浏览器运行原理</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><!--]--></div></section></div><div class="group" data-v-eec6a639><section class="VPSidebarItem level-0 collapsible" data-v-eec6a639 data-v-66b47b62><div class="item" role="button" data-v-66b47b62><div class="indicator" data-v-66b47b62></div><a class="VPLink link" data-v-66b47b62 data-v-8dd200cc><!--[--><h2 class="text" data-v-66b47b62>学习笔记</h2><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" viewbox="0 0 24 24" class="caret-icon" data-v-66b47b62><path d="M9,19c-0.3,0-0.5-0.1-0.7-0.3c-0.4-0.4-0.4-1,0-1.4l5.3-5.3L8.3,6.7c-0.4-0.4-0.4-1,0-1.4s1-0.4,1.4,0l6,6c0.4,0.4,0.4,1,0,1.4l-6,6C9.5,18.9,9.3,19,9,19z"></path></svg></div></div><div class="items" data-v-66b47b62><!--[--><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></div><a class="VPLink link link" href="/note/JavaScript.html" data-v-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>JavaScript基础</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></div><a class="VPLink link link" href="/note/CSS.html" data-v-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>CSS基础</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></div><a class="VPLink link link" href="/note/JavaScriptEnhanced.html" data-v-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>JavaScript进阶</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></div><a class="VPLink link link" href="/note/Front-end%20Engineering.html" data-v-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>前端工程化</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-66b47b62 data-v-66b47b62><div class="item" data-v-66b47b62><div class="indicator" data-v-66b47b62></div><a class="VPLink link link" href="/note/SSR.html" data-v-66b47b62 data-v-8dd200cc><!--[--><p class="text" data-v-66b47b62>服务端渲染</p><!--]--><!----></a><div class="caret" role="button" data-v-66b47b62><!----></div></div><!----></div><!--]--></div></section></div><!--]--><!--[--><!--]--></nav></aside><div class="VPContent has-sidebar" id="VPContent" data-v-3af881ed data-v-e31feb0e><div class="VPDoc has-sidebar has-aside" data-v-e31feb0e data-v-30dabfe4><div class="container" data-v-30dabfe4><div class="aside" data-v-30dabfe4><div class="aside-curtain" data-v-30dabfe4></div><div class="aside-container" data-v-30dabfe4><div class="aside-content" data-v-30dabfe4><div class="VPDocAside" data-v-30dabfe4 data-v-7110f5c9><!--[--><!--]--><!--[--><!--]--><div class="VPDocAsideOutline" data-v-7110f5c9 data-v-9c6f1a55><div class="content" data-v-9c6f1a55><div class="outline-marker" data-v-9c6f1a55></div><div class="outline-title" data-v-9c6f1a55>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-9c6f1a55><span class="visually-hidden" id="doc-outline-aria-label" data-v-9c6f1a55> Table of Contents for current page </span><ul class="root" data-v-9c6f1a55 data-v-02afc12d><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-7110f5c9></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-30dabfe4><div class="content-container" data-v-30dabfe4><!--[--><!--]--><main class="main" data-v-30dabfe4><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-30dabfe4><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-hidden="true">#</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-hidden="true">#</a></h2><p>函数在调用时, Javascript会默认为this绑定一个值</p><div class="language- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">// 定义一个函数</span></span>
<span class="line"><span style="color:#A6ACCD;">function foo() {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">// 1. 直接调用</span></span>
<span class="line"><span style="color:#A6ACCD;">foo() // Window</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">// 2. 绑定对象调用</span></span>
<span class="line"><span style="color:#A6ACCD;">const obj = { name: &#39;ziu&#39;, aaa: foo }</span></span>
<span class="line"><span style="color:#A6ACCD;">obj.aaa() // obj</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">// 3. 通过call/apply调用</span></span>
<span class="line"><span style="color:#A6ACCD;">foo.call(&#39;Ziu&#39;) // String {&#39;Ziu&#39;}</span></span>
<span class="line"><span style="color:#A6ACCD;"></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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">function foo() {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">foo() // Window</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">const obj = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &#39;ziu&#39;,</span></span>
<span class="line"><span style="color:#A6ACCD;">  bar: function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">    console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;"> }</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">obj.bar() // obj</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">const baz = obj.bar</span></span>
<span class="line"><span style="color:#A6ACCD;">baz() // Window</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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-hidden="true">#</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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">function foo() {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">  this.name = &#39;Ziu&#39;</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">foo() // 直接调用的话 this为Window</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">new foo() // 通过new关键字调用 则this指向空对象</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">obj.foo = foo // 绑定</span></span>
<span class="line"><span style="color:#A6ACCD;">obj.foo() // 调用</span></span>
<span class="line"><span style="color:#A6ACCD;"></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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">var obj = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &#39;Ziu&#39;</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">function foo() {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">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 style="color:#A6ACCD;">foo.call(123) // foo的this被绑定到了 Number { 123 } 上</span></span>
<span class="line"><span style="color:#A6ACCD;">foo.call(&quot;ziu&quot;) // 绑定到了 String { &quot;ziu&quot; } 上</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</a></h4><p>当我们直接使用类似：</p><div class="language- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">&quot;ZiuChen&quot;.length // String.length</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">function foo(name, age, height) {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">foo(&#39;Ziu&#39;, 18, 1.88)</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">foo.apply(&#39;targetThis&#39;, &#39;Ziu&#39;, 18, 1.88)</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">foo.call(&#39;targetThis&#39;, [&#39;Ziu&#39;, 18, 1.88])</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">function foo(...args) {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(args)</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">​</span></span>
<span class="line"><span style="color:#A6ACCD;">foo(&quot;Ziu&quot;, 18, 1.88) // [&quot;Ziu&quot;, 18, 1.88]</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">// 将obj绑定到foo上</span></span>
<span class="line"><span style="color:#A6ACCD;">const fun = foo.bind(obj)</span></span>
<span class="line"><span style="color:#A6ACCD;">// 在后续每次调用foo时, foo内的this都将指向obj</span></span>
<span class="line"><span style="color:#A6ACCD;">fun() // obj</span></span>
<span class="line"><span style="color:#A6ACCD;">fun() // obj</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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-hidden="true">#</a></h3><p>在箭头函数中是没有<code>this</code>的：</p><div class="language- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">const foo = () =&gt; {</span></span>
<span class="line"><span style="color:#A6ACCD;">  console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">foo() // window</span></span>
<span class="line"><span style="color:#A6ACCD;">console.log(this) // window</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</a></h3><p>检查以下代码：</p><div class="language- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">const obj = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &quot;obj&quot;,</span></span>
<span class="line"><span style="color:#A6ACCD;">  foo: function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">    const bar = () =&gt; {</span></span>
<span class="line"><span style="color:#A6ACCD;">      console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">   }</span></span>
<span class="line"><span style="color:#A6ACCD;">    return bar</span></span>
<span class="line"><span style="color:#A6ACCD;"> }</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">const fn = obj.foo()</span></span>
<span class="line"><span style="color:#A6ACCD;">fn() // obj</span></span>
<span class="line"><span style="color:#A6ACCD;"></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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">const obj = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &quot;obj&quot;,</span></span>
<span class="line"><span style="color:#A6ACCD;">  foo: () =&gt; {</span></span>
<span class="line"><span style="color:#A6ACCD;">    const bar = () =&gt; {</span></span>
<span class="line"><span style="color:#A6ACCD;">      console.log(this)</span></span>
<span class="line"><span style="color:#A6ACCD;">   }</span></span>
<span class="line"><span style="color:#A6ACCD;">    return bar</span></span>
<span class="line"><span style="color:#A6ACCD;"> }</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">const fn = obj.foo()</span></span>
<span class="line"><span style="color:#A6ACCD;">fn() // Window</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-hidden="true">#</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-hidden="true">#</a></h2><div class="language- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">var name = &#39;window&#39;</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">var person = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &#39;person&#39;,</span></span>
<span class="line"><span style="color:#A6ACCD;">  sayName: function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">    console.log(this.name)</span></span>
<span class="line"><span style="color:#A6ACCD;"> }</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">function sayName() {</span></span>
<span class="line"><span style="color:#A6ACCD;">  var sss = person.sayName</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">  sss() // 默认绑定: window</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">  person.sayName();  // 隐式绑定: person</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;"> (person.sayName)() // 隐式绑定: person, 本质与上一行代码相同</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;"> ;(person.sayName = person.sayName)() // 间接调用: window</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">sayName()</span></span>
<span class="line"><span style="color:#A6ACCD;"></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- line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang"></span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#A6ACCD;">var name = &#39;window&#39;</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">var person1 = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &#39;person1&#39;,</span></span>
<span class="line"><span style="color:#A6ACCD;">  foo1: function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">    console.log(this.name)</span></span>
<span class="line"><span style="color:#A6ACCD;"> },</span></span>
<span class="line"><span style="color:#A6ACCD;">  foo2: () =&gt; console.log(this.name),</span></span>
<span class="line"><span style="color:#A6ACCD;">  foo3: function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">    return function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">      console.log(this.name)</span></span>
<span class="line"><span style="color:#A6ACCD;">   }</span></span>
<span class="line"><span style="color:#A6ACCD;"> },</span></span>
<span class="line"><span style="color:#A6ACCD;">  foo4: function () {</span></span>
<span class="line"><span style="color:#A6ACCD;">    return () =&gt; console.log(this.name)</span></span>
<span class="line"><span style="color:#A6ACCD;"> }</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">var person2 = {</span></span>
<span class="line"><span style="color:#A6ACCD;">  name: &#39;person2&#39;</span></span>
<span class="line"><span style="color:#A6ACCD;">}</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo1() // 隐式绑定: person1</span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo1.call(person2) // 显式绑定: person2</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo2() // 上层作用域: window</span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo2.call(person2) // 上层作用域: window</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo3()() // 默认绑定: window</span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo3.call(person2)() // 默认绑定: window</span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo3().call(person2) // 显式绑定: person2</span></span>
<span class="line"><span style="color:#A6ACCD;"></span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo4()() // 隐式绑定: person1</span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo4.call(person2)() // 显式绑定: person2</span></span>
<span class="line"><span style="color:#A6ACCD;">person1.foo4().call(person2) // 隐式绑定: person1</span></span>
<span class="line"><span style="color:#A6ACCD;"></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-30dabfe4 data-v-cc0f9778><div class="edit-info" data-v-cc0f9778><div class="edit-link" data-v-cc0f9778><a class="VPLink link 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-cc0f9778 data-v-8dd200cc><!--[--><svg xmlns="http://www.w3.org/2000/svg" viewbox="0 0 24 24" class="edit-link-icon" data-v-cc0f9778><path d="M18,23H4c-1.7,0-3-1.3-3-3V6c0-1.7,1.3-3,3-3h7c0.6,0,1,0.4,1,1s-0.4,1-1,1H4C3.4,5,3,5.4,3,6v14c0,0.6,0.4,1,1,1h14c0.6,0,1-0.4,1-1v-7c0-0.6,0.4-1,1-1s1,0.4,1,1v7C21,21.7,19.7,23,18,23z"></path><path d="M8,17c-0.3,0-0.5-0.1-0.7-0.3C7,16.5,6.9,16.1,7,15.8l1-4c0-0.2,0.1-0.3,0.3-0.5l9.5-9.5c1.2-1.2,3.2-1.2,4.4,0c1.2,1.2,1.2,3.2,0,4.4l-9.5,9.5c-0.1,0.1-0.3,0.2-0.5,0.3l-4,1C8.2,17,8.1,17,8,17zM9.9,12.5l-0.5,2.1l2.1-0.5l9.3-9.3c0.4-0.4,0.4-1.1,0-1.6c-0.4-0.4-1.2-0.4-1.6,0l0,0L9.9,12.5z M18.5,2.5L18.5,2.5L18.5,2.5z"></path></svg> Edit this page on GitHub<!--]--><!----></a></div><div class="last-updated" data-v-cc0f9778><p class="VPLastUpdated" data-v-cc0f9778 data-v-de6a4e1f>Updated Date: <time datetime="2023-02-21T11:31:03.000Z" data-v-de6a4e1f></time></p></div></div><div class="prev-next" data-v-cc0f9778><div class="pager" data-v-cc0f9778><a class="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-cc0f9778><span class="desc" data-v-cc0f9778>Previous page</span><span class="title" data-v-cc0f9778>一文读懂伪类与伪元素</span></a></div><div class="has-prev pager" data-v-cc0f9778><a class="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-cc0f9778><span class="desc" data-v-cc0f9778>Next page</span><span class="title" data-v-cc0f9778>从0实现一个年度报告</span></a></div></div></footer><!--[--><!--]--></div></div></div></div></div><footer class="VPFooter has-sidebar" data-v-3af881ed data-v-8b655284><div class="container" data-v-8b655284><p class="message" data-v-8b655284>Released under the MIT License.</p><p class="copyright" data-v-8b655284>Copyright © 2019-present Ziu Chen</p></div></footer><!--[--><!--]--></div></div>
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