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<div><a href="#" class="skipToContent_1oUP">Skip to main content</a></div><nav class="navbar navbar--fixed-top navbarHideable_2qcr"><div class="navbar__inner"><div class="navbar__items"><button aria-label="Navigation bar toggle" class="navbar__toggle clean-btn" type="button" tabindex="0"><svg width="30" height="30" viewBox="0 0 30 30" aria-hidden="true"><path stroke="currentColor" stroke-linecap="round" stroke-miterlimit="10" stroke-width="2" d="M4 7h22M4 15h22M4 23h22"></path></svg></button><a class="navbar__brand" href="/awesome-interview/"><img src="/awesome-interview/img/badge.svg" alt="HZFE" class="themedImage_1VuW themedImage--light_3UqQ navbar__logo"><img src="/awesome-interview/img/badge.svg" alt="HZFE" class="themedImage_1VuW themedImage--dark_hz6m navbar__logo"><b class="navbar__title">剑指前端 Offer</b></a></div><div class="navbar__items navbar__items--right"><a href="https://github.com/hzfe/awesome-interview" target="_blank" rel="noopener noreferrer" class="navbar__item navbar__link"><span>GitHub<svg width="13.5" height="13.5" aria-hidden="true" viewBox="0 0 24 24" class="iconExternalLink_3J9K"><path fill="currentColor" d="M21 13v10h-21v-19h12v2h-10v15h17v-8h2zm3-12h-10.988l4.035 4-6.977 7.07 2.828 2.828 6.977-7.07 4.125 4.172v-11z"></path></svg></span></a><div class="searchBox_1Doo"><button type="button" class="DocSearch DocSearch-Button" aria-label="Search"><span class="DocSearch-Button-Container"><svg width="20" height="20" class="DocSearch-Search-Icon" viewBox="0 0 20 20"><path d="M14.386 14.386l4.0877 4.0877-4.0877-4.0877c-2.9418 2.9419-7.7115 2.9419-10.6533 0-2.9419-2.9418-2.9419-7.7115 0-10.6533 2.9418-2.9419 7.7115-2.9419 10.6533 0 2.9419 2.9418 2.9419 7.7115 0 10.6533z" stroke="currentColor" fill="none" fill-rule="evenodd" stroke-linecap="round" stroke-linejoin="round"></path></svg><span class="DocSearch-Button-Placeholder">Search</span></span><span class="DocSearch-Button-Keys"></span></button></div></div></div><div role="presentation" class="navbar-sidebar__backdrop"></div></nav><div class="main-wrapper docs-wrapper docs-doc-page"><div class="docPage_31aa"><button class="clean-btn backToTopButton_35hR" type="button"><svg viewBox="0 0 24 24" width="28"><path d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z" fill="currentColor"></path></svg></button><aside class="docSidebarContainer_3Kbt"><div class="sidebar_15mo sidebarWithHideableNavbar_267A"><a tabindex="-1" class="sidebarLogo_3h0W" href="/awesome-interview/"><img src="/awesome-interview/img/badge.svg" alt="HZFE" class="themedImage_1VuW themedImage--light_3UqQ"><img src="/awesome-interview/img/badge.svg" alt="HZFE" class="themedImage_1VuW themedImage--dark_hz6m"><b>剑指前端 Offer</b></a><nav class="menu thin-scrollbar menu_Bmed menuWithAnnouncementBar_2WvA"><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" href="/awesome-interview/about">关于我们</a></li><li class="menu__list-item"><a class="menu__link" href="/awesome-interview/">前言</a></li><li class="menu__list-item"><a class="menu__link menu__link--sublist menu__link--active" href="#">模拟题一</a><ul style="display:block;overflow:visible;height:auto" class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/awesome-interview/book1/browser-cross-origin">浏览器：浏览器跨域</a></li><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/awesome-interview/book1/browser-repain-reflow">浏览器：浏览器的重排重绘</a></li><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/awesome-interview/book1/engineer-webpack-workflow">工程化：Webpack 工作流程</a></li><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/awesome-interview/book1/frame-vue-data-binding">框架：Vue 的数据绑定机制</a></li><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/awesome-interview/book1/frame-vue-computed-watch">框架：Vue 的 computed 和 watch 的区别</a></li><li class="menu__list-item"><a class="menu__link menu__link--active" aria-current="page"
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">var</span><span class="token plain"> baz </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">foo</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token function" style="color:#d73a49">baz</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// hzfe</span></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p>在这个例子中，函数 bar 作为返回值返回后，在自己定义的词法作用域以外的地方执行。一般来说，在函数 foo 执行后，通常会期待函数 foo 的整个内部作用域被引擎回收机制销毁。而闭包可以阻止这件事情的发生。事实上内部作用域依然存在，因为函数 bar 本身在使用，所以并不会被回收。</p><p>在 JavaScript 中，<strong>每当创建一个函数，闭包就会在函数创建的同时被创建出来</strong>。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="3-闭包的应用"></a>3. 闭包的应用<a class="hash-link" href="#3-闭包的应用" title="Direct link to heading">#</a></h3><p>无论何时何地，如果<strong>将函数作为返回值，就会看到闭包在这些函数中的应用</strong>。在定时器，事件监听器，ajax 请求，跨窗口通信，web workers 或者任何其他的异步/同步任务中，只要<strong>使用了回调函数，实际上就是使用闭包</strong>。使用闭包的例子可以参考<a href="/awesome-interview/book2/coding-throttle-debounce">实现节流防抖函数</a>。</p><p><strong>TIPS: 闭包与执行函数关系</strong></p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly javascript"><pre tabindex="0" class="prism-code language-javascript codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">var</span><span class="token plain"> a </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"hzfe"</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token constant" style="color:#36acaa">IIFE</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token console class-name">console</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">log</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">a</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punct
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2021-09-21 16:46:32 +08:00
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它在定义时所在的作用域中执行，而非外部作用域。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithHideOnScrollNavbar_3R7-" id="参考资料"></a>参考资料<a class="hash-link" href="#参考资料" title="Direct link to heading">#</a></h2><ol><li><a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Closures" target="_blank" rel="noopener noreferrer">闭包 MDN</a></li><li><a href="/awesome-interview/book2/browser-garbage">垃圾回收机制</a></li><li><a href="https://github.com/getify/You-Dont-Know-JS" target="_blank" rel="noopener noreferrer">你不知道的 JavaScript（上卷）</a></li><li><a href="https://zh.wikipedia.org/wiki/%E9%97%AD%E5%8C%85_(%E8%AE%A1%E7%AE%97%E6%9C%BA%E7%A7%91%E5%AD%A6)" target="_blank" rel="noopener noreferrer">维基百科-闭包</a></li></ol></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages navigation"><div class="pagination-nav__item"><a class="pagination-nav__link" href="/awesome-interview/book1/frame-vue-computed-watch"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« 框架：Vue 的 computed 和 watch 的区别</div></a></div><div class="pagination-nav__item pagination-nav__item--next"><a class="pagination-nav__link" href="/awesome-interview/book1/js-module-specs"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">基础：前端模块化规范 »</div></a></div></nav></div></div><div class="col col--3"><div class="tableOfContents_35-E thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#相关问题" class="table-of-contents__link">相关问题</a></li><li><a href="#回答关键点" class="table-of-contents__link">回答关键点</a></li><li><a href="#知识点深入" class="table-of-contents__link">知识点深入</a><ul><li><a href="#1-作用域" class="table-of-contents__link">1. 作用域</a></li><li><a href="#2-什么是闭包" class="table-of-contents__link">2. 什么是闭包</a></li><li><a href="#3-闭包的应用" class="table-of-contents__link">3. 闭包的应用</a></li></ul></li><li><a href="#参考资料" class="table-of-contents__link">参考资料</a></li></ul></div></div></div><div class="row"><div class="col"><div class="react-utterences"><div>Loading script...</div></div></div><div class="col col--3"></div></div></div></main></div></div></div>
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