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class="breadcrumbs__link">模拟题一</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">基础：闭包的作用和原理</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>闭包的作用和原理</h1><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="相关问题">相关问题<a href="#相关问题" class="hash-link" aria-label="Direct link to 相关问题" title="Direct link to 相关问题"></a></h2><ul><li>什么是闭包</li><li>闭包的应用</li></ul><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="回答关键点">回答关键点<a href="#回答关键点" class="hash-link" aria-label="Direct link to 回答关键点" title="Direct link to 回答关键点"></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 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="知识点深入">知识点深入<a href="#知识点深入" class="hash-link" aria-label="Direct link to 知识点深入" title="Direct link to 知识点深入"></a></h2><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="1-作用域">1. 作用域<a href="#1-作用域" class="hash-link" aria-label="Direct link to 1. 作用域" title="Direct link to 1. 作用域"></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 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="2-什么是闭包">2. 什么是闭包<a href="#2-什么是闭包" class="hash-link" aria-label="Direct link to 2. 什么是闭包" title="Direct link to 2. 什么是闭包"></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="language-javascript codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-javascript codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><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><br></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><br></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><br></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><br></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><br></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><br></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><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></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><br></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><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></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 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="3-闭包的应用">3. 闭包的应用<a href="#3-闭包的应用" class="hash-link" aria-label="Direct link to 3. 闭包的应用" title="Direct link to 3. 闭包的应用"></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="language-javascript codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-javascript codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><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><br></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><br></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><br></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><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></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 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="参考资料">参考资料<a href="#参考资料" class="hash-link" aria-label="Direct link to 参考资料" title="Direct link to 参考资料"></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"><a class="pagination-nav__link pagination-nav__link--prev" 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><a class="pagination-nav__link pagination-nav__link--next" 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></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#相关问题" class="table-of-contents__link toc-highlight">相关问题</a></li><li><a href="#回答关键点" class="table-of-contents__link toc-highlight">回答关键点</a></li><li><a href="#知识点深入" class="table-of-contents__link toc-highlight">知识点深入</a><ul><li><a href="#1-作用域" class="table-of-contents__link toc-highlight">1. 作用域</a></li><li><a href="#2-什么是闭包" class="table-of-contents__link toc-highlight">2. 什么是闭包</a></li><li><a href="#3-闭包的应用" class="table-of-contents__link toc-highlight">3. 闭包的应用</a></li></ul></li><li><a href="#参考资料" class="table-of-contents__link toc-highlight">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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