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class="row"><div class="col docItemCol_3FnS"><div class="docItemContainer_33ec"><article><div class="tocCollapsible_1PrD tocMobile_3Hoh"><button type="button" class="clean-btn tocCollapsibleButton_2O1e">On this page</button></div><div class="markdown"><header><h1>闭包的作用和原理</h1></header><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="相关问题"></a>相关问题<a class="hash-link" href="#相关问题" title="Direct link to heading">#</a></h2><ul><li>什么是闭包</li><li>闭包的应用</li></ul><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="回答关键点"></a>回答关键点<a class="hash-link" href="#回答关键点" title="Direct link to heading">#</a></h2><p><code>作用域</code> <code>引用</code> <code>函数</code></p><p>作用：能够在函数定义的作用域外，使用函数定义作用域内的局部变量，并且不会污染全局。</p><p>原理：基于词法作用域链和垃圾回收机制，通过维持函数作用域的引用，让函数作用域可以在当前作用域外被访问到。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="知识点深入"></a>知识点深入<a class="hash-link" href="#知识点深入" title="Direct link to heading">#</a></h2><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="1-作用域"></a>1. 作用域<a class="hash-link" href="#1-作用域" title="Direct link to heading">#</a></h3><ul><li><strong>作用域</strong>：用于确定在何处以及如何查找变量（标识符）的一套规则。</li><li><strong>词法作用域</strong>：词法作用域是<strong>定义在词法阶段的作用域</strong>。词法作用域是由写代码时将代码和块作用域写在哪里来决定的，因此当词法作用域处理代码是会保持作用域不变(大部分情况)。</li><li><strong>块作用域</strong>：指的是变量和函数不仅可以属于所处的作用域，也可以属于某个代码块（通常用{}包裹）。常见的块级作用域有 with，try/catch，let，const 等。</li><li><strong>函数作用域</strong>：属于这个函数的全部变量都可以在整个函数范围内使用及复用（包括嵌套作用域）。</li><li><strong>作用域链</strong>：查找变量时，先从当前作用域开始查找，如果没有找到，就会到父级(词法层面上的父级)作用域中查找，一直找到全局作用域。作用域链正是包含这些作用域的列表。</li></ul><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="2-什么是闭包"></a>2. 什么是闭包<a class="hash-link" href="#2-什么是闭包" title="Direct link to heading">#</a></h3><blockquote><p>当函数可以记住并访问所在的词法作用域时，就产生了闭包，即使是函数在当前词法作用域外执行。 ——《你不知道的 JavaScript》</p></blockquote><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">function</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 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 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 keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">bar</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 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 plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> bar</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 plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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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 anchorWithStickyNavbar_31ik" 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 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 punctuation" style="color:#393A34">)</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></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p>通常认为立即执行函数（IIFE）是典型的观察闭包的典型例子，但严格来说并不是。<strong>虽然创建了闭包，但没有体现出闭包的作用</strong>。因为函数并不是在它本身的词法作用域以外执行的。
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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 anchorWithStickyNavbar_31ik" 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></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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