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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>Babel 的原理</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>Babel 是什么</li><li>Babel 有什么用</li><li>压缩代码如何实现</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>JS 编译器</code> <code>AST</code> <code>插件系统</code></p><p>Babel 是 <strong>JavaScript 编译器</strong>：他能让开发者在开发过程中，直接使用各类方言（如 TS、Flow、JSX）或新的语法特性，而不需要考虑运行环境，因为 Babel 可以做到按需转换为低版本支持的代码；Babel 内部原理是将 JS 代码转换为 <strong>AST</strong>，对 AST 应用各种插件进行处理，最终输出编译后的 JS 代码。</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-ast-抽象语法树"></a>1. AST 抽象语法树<a class="hash-link" href="#1-ast-抽象语法树" title="Direct link to heading">#</a></h3><p><strong>简单定义</strong>：以树的形式来表现编程语言的语法结构。</p><p><img src="https://user-images.githubusercontent.com/17002181/124874255-1742bd80-dffa-11eb-9fcf-2a2f9e130334.png" alt="image"></p><p>利用在线 <a href="https://astexplorer.net/" target="_blank" rel="noopener noreferrer">playground</a> 调试，可以对 AST 有个直观感受：生成的树有多个节点，节点有不同的类型，不同类型节点有不同的属性。</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly js"><pre tabindex="0" class="prism-code language-js 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">const</span><span class="token plain"> custom </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></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p><img src="https://user-images.githubusercontent.com/17002181/124893664-7fe76580-e00d-11eb-8937-a6b7719643f4.png" alt="image"></p><p>AST 是源代码的高效表示，能便捷的表示大多数编程语言的结构。适用于做代码分析或转换等需求。之所以用树来进行分析或转换，是因为树能使得程序中的每一节点恰好被访问一次（前序或后续遍历）。</p><p><strong>常见使用场景</strong>：代码压缩混淆功能可以借助 AST 来实现：分析 AST，基于各种规则进行优化（如 IF 语句优化；移除不可访问代码；移除 debugger 等），从而生成更小的 AST 树，最终输出精简的代码结果。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="2-babel-编译流程"></a>2. Babel 编译流程<a class="hash-link" href="#2-babel-编译流程" title="Direct link to heading">#</a></h3><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="三大步骤"></a>三大步骤<a class="hash-link" href="#三大步骤" title="Direct link to heading">#</a></h4><p><img src="https://user-images.githubusercontent.com/17002181/124876479-9f29c700-dffc-11eb-9eef-a4bc85ebb6be.png" alt="image"></p><ol><li><p><strong>解析阶段</strong>：Babel 默认使用 <strong>@babel/parser</strong> 将代码转换为 AST。解析一般分为两个阶段：词法分析和语法分析。</p><ul><li><strong>词法分析</strong>：对输入的字符序列做标记化(tokenization)操作。</li><li><strong>语法分析</strong>：处理标记与标记之间的关系，最终形成一颗完整的 AST 结构。</li></ul></li><li><p><strong>转换阶段</strong>：Babel 使用 <strong>@babel/traverse</strong> 提供的方法对 AST 进行深度优先遍历，调用插件对关注节点的处理函数，按需对 AST 节点进行增删改操作。</p></li><li><p><strong>生成阶段</strong>：Babel 默认使用 <strong>@babel/generator</strong> 将上一阶段处理后的 AST 转换为代码字符串。</p></li></ol><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="3-babel-插件系统"></a>3. Babel 插件系统<a class="hash-link" href="#3-babel-插件系统" title="Direct link to heading">#</a></h3><p>Babel 的核心模块 @babel/core，@babel/parser，@babel/traverse 和 @babel/generator 提供了完整的编译流程。而具体的转换逻辑需要插件来完成。</p><p>在使用 Babel 时，我们可通过配置文件指定 plugin 和 preset。而 preset 可以是 plugin 和 preset 以及其他配置的集合。Babel 会递归读取 preset，最终获取一个大的 plugins 数组，用于后续使用。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="常见-presets"></a>常见 presets<a class="hash-link" href="#常见-presets" title="Direct link to heading">#</a></h4><ul><li>@babel/preset-env</li><li>@babel/preset-typescript</li><li>@babel/preset-react</li><li>@babel/preset-flow</li></ul><p>最常见的 @babel/preset-env 预设，包含了<a href="https://github.com/babel/babel/blob/master/packages/babel-compat-data/data/plugins.json" target="_blank" rel="noopener noreferrer">一组</a>最新浏览器已支持的 ES 语法特性，并且可以通过配置目标运行环境范围，自动按需引入插件。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="编写-babel-插件"></a>编写 Babel 插件<a class="hash-link" href="#编写-babel-插件" title="Direct link to heading">#</a></h4><p>Babel 插件的写法是借助<strong>访问者模式</strong>（Visitor Pattern）对关注的节点定义处理函数。参考一个简单 Babel 插件例子：</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly js"><pre tabindex="0" class="prism-code language-js codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token plain">module</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method-variable function-variable method function property-access" style="color:#d73a49">exports</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </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 control-flow" style="color:#00009f">return</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 function" style="color:#d73a49">pre</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 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 comment" style="color:#999988;font-style:italic">// 在 visitor 下挂载各种感兴趣的节点类型的监听方法</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> visitor</span><span class="token operator" 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 comment" style="color:#999988;font-style:italic">/**</span></span><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic"> * 对 Identify 类型的节点进行处理</span></span><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic"> * @param {NodePath} path</span></span><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic"> */</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function maybe-class-name" style="color:#d73a49">Identifier</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">path</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"> path</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">node</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">name</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> path</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">node</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">name</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">toUpperCase</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 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 plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function" style="color:#d73a49">post</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 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 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></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p>使用该 Babel 插件的效果如下：</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly js"><pre tabindex="0" class="prism-code language-js codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic">// input</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 comment" style="color:#999988;font-style:italic">// index.js</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">hzfe</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 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 comment" style="color:#999988;font-style:italic">// .babelrc</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 string" style="color:#e3116c">"plugins"</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token string" style="color:#e3116c">"babel-plugin-yourpluginname"</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></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly js"><pre tabindex="0" class="prism-code language-js codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic">// output</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 constant" style="color:#36acaa">HZFE</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 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><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="深入-babel-转换阶段"></a>深入 Babel 转换阶段<a class="hash-link" href="#深入-babel-转换阶段" title="Direct link to heading">#</a></h4><p>在转换阶段，Babel 的相关方法会获得一个插件数组变量，用于后续的操作。插件结构可参考以下接口。</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly ts"><pre tabindex="0" class="prism-code language-ts 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">interface</span><span class="token plain"> </span><span class="token class-name maybe-class-name">Plugin</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"> key</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token builtin">string</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">undefined</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">null</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"> post</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</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"> pre</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</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"> visitor</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Object</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"> parserOverride</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</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"> generatorOverride</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</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 comment" style="color:#999988;font-style:italic">// ...</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></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p><strong>转换阶段</strong>，Babel 会按以下顺序执行。详细逻辑可查看<a href="https://github.com/babel/babel/blob/a647b9ea6bdd510f0d80b58dbea66828016ffe00/packages/babel-core/src/transformation/index.ts#L76" target="_blank" rel="noopener noreferrer">源码</a>：</p><ol><li>执行所有插件的 pre 方法。</li><li>按需执行 visitor 中的方法。</li><li>执行所有插件的 post 方法。</li></ol><p>一般来说，写 Babel 插件主要使用到的是 visitor 对象，这个 visitor 对象中会书写对于关注的 AST 节点的处理逻辑。而上面执行顺序中的第二步所指的 visitor 对象，是整合自各插件的 visitor，最终形成一个大的 visitor 对象，大致的数据结构可参考以下接口：</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly ts"><pre tabindex="0" class="prism-code language-ts codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic">// 书写插件时的 visitor 结构</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">interface</span><span class="token plain"> </span><span class="token class-name maybe-class-name">VisitorInPlugin</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 punctuation" style="color:#393A34">[</span><span class="token maybe-class-name">ASTNodeTypeName</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token builtin">string</span><span class="token punctuation" style="color:#393A34">]</span><span class="token operator" 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 operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token operator" 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"> enter</span><span class="token operator" style="color:#393A34">?</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</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"> exit</span><span class="token operator" style="color:#393A34">?</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</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 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 comment" style="color:#999988;font-style:italic">// babel 最终整合的 visitor 结构</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">interface</span><span class="token plain"> </span><span class="token class-name maybe-class-name">VisitorInTransform</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 punctuation" style="color:#393A34">[</span><span class="token maybe-class-name">ASTNodeTypeName</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token builtin">string</span><span class="token punctuation" style="color:#393A34">]</span><span class="token operator" 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 comment" style="color:#999988;font-style:italic">// 不同插件对相同节点的处理会合并为数组</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> enter</span><span class="token operator" style="color:#393A34">?</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</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"> exit</span><span class="token operator" style="color:#393A34">?</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Function</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 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></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p>在对 AST 进行<strong>深度优先遍历</strong>的过程中，会创建 TraversalContext 对象来把控对 NodePath 节点的访问，访问时调用对节点所定义的处理方法，从而实现按需执行 visitor 中的方法。详细实现请看 <a href="https://github.com/babel/babel/tree/bc1b9537b00499f462aa3ac0d49e30314a66f413/packages/babel-traverse" target="_blank" rel="noopener noreferrer">babel-traverse</a> 中的源码。</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://en.wikipedia.org/wiki/Abstract_syntax_tree" target="_blank" rel="noopener noreferrer">AST</a></li><li><a href="https://github.com/jamiebuilds/babel-handbook" target="_blank" rel="noopener noreferrer">Babel-handbook</a></li><li><a href="https://github.com/estree/estree" target="_blank" rel="noopener noreferrer">estree</a></li><li><a href="https://en.wikipedia.org/wiki/Visitor_pattern" target="_blank" rel="noopener noreferrer">访问者模式</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/book2/browser-garbage"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« 浏览器：垃圾回收机制</div></a></div><div class="pagination-nav__item pagination-nav__item--next"><a class="pagination-nav__link" href="/awesome-interview/book2/frame-react-fiber"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">框架：React Fiber 的作用和原理 »</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-ast-抽象语法树" class="table-of-contents__link">1. 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