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fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">模拟题三</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">工程化：谈下 webpack loader 的机制</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>谈下 webpack loader 的机制</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>webpack loader 是如何工作的</li><li>如何编写 webpack loader</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>转换</code> <code>生命周期</code> <code>chunk</code></p><p>webpack 本身只能处理 JavaScript 和 JSON 文件，而 loader 为 webpack 添加了处理其他类型文件的能力。loader 将其他类型的文件转换成有效的 webpack modules（如 ESmodule、CommonJS、AMD），webpack 能消费这些模块，并将其添加到依赖关系图中。</p><p>loader 本质上是一个函数，该函数对接收到的内容进行转换，返回转换后的结果。</p><p>常见的 loader 有：</p><ul><li>raw-loader：加载文件原始内容。</li><li>file-loader：将引用文件输出到目标文件夹中，在代码中通过相对路径引用输出的文件。</li><li>url-loader：和 file-loader 类似，但是能在文件很小的情况下以 base64 的方式将文件内容注入到代码中。</li><li>babel-loader：将 ES 较新的语法转换为浏览器可以兼容的语法。</li><li>style-loader：将 CSS 代码注入到 JavaScript 中，通过 DOM 操作加载 CSS。</li><li>css-loader：加载 CSS，支持模块化、压缩、文件导入等特性。</li></ul><p>使用 loader 的方式主要有两种：</p><ol><li>在 webpack.config.js 文件中配置，通过在 module.rules 中使用 test 匹配要转换的文件类型，使用 use 指定要使用的 loader。</li></ol><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">module</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">exports</span><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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">module</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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">rules</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 punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">test</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-source language-regex" style="color:#36acaa">\.ts$</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">use</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"ts-loader"</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><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 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><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><ol start="2"><li>内联使用</li></ol><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports maybe-class-name">Styles</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"style-loader!css-loader?modules!./styles.css"</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><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-编写-webpack-loader">1. 编写 webpack loader<a href="#1-编写-webpack-loader" class="hash-link" aria-label="Direct link to 1. 编写 webpack loader" title="Direct link to 1. 编写 webpack loader"></a></h3><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="11-同步-loader">1.1 同步 loader<a href="#11-同步-loader" class="hash-link" aria-label="Direct link to 1.1 同步 loader" title="Direct link to 1.1 同步 loader"></a></h4><p>同步转换内容后，可以通过 return 或调用 this.callback 返回结果。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">export</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">default</span><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">loader</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">content</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> map</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> meta</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 control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">someSyncOperation</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">content</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><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>通过 this.callback 可以返回除内容以外的其他信息（如 sourcemap）。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">export</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">default</span><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">loader</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">content</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> map</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> meta</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">this</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">someSyncOperation</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">content</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> map</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> meta</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 keyword control-flow" style="color:#00009f">return</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 当调用 callback() 时，始终返回 undefined</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><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><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="12-异步-loader">1.2 异步 loader<a href="#12-异步-loader" class="hash-link" aria-label="Direct link to 1.2 异步 loader" title="Direct link to 1.2 异步 loader"></a></h4><p>通过 this.async 可以获取异步操作的回调函数，并在回调函数中返回结果。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">export</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">default</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 parameter">content</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> map</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> meta</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">const</span><span class="token plain"> callback </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">this</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">async</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">someAsyncOperation</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">content</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 parameter">err</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> result</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> sourceMaps</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> meta</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" 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 control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">err</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">err</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">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> result</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> sourceMaps</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> meta</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 plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></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>除非计算很小，否则对于 Node.js 这种单线程环境，尽可能使用异步 loader。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="13-loader-开发辅助工具及-loadercontext">1.3 loader 开发辅助工具及 loaderContext<a href="#13-loader-开发辅助工具及-loadercontext" class="hash-link" aria-label="Direct link to 1.3 loader 开发辅助工具及 loaderContext" title="Direct link to 1.3 loader 开发辅助工具及 loaderContext"></a></h4><p><code>loader-utils</code> 与 <code>schema-utils</code>，可以使获取及验证传递给 loader 的参数的工作简单化。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports punctuation" style="color:#393A34">{</span><span class="token imports"> getOptions </span><span class="token imports punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"loader-utils"</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 module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports punctuation" style="color:#393A34">{</span><span class="token imports"> validate </span><span class="token imports punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"schema-utils"</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">const</span><span class="token plain"> schema </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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">type</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"object"</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 literal-property property" style="color:#36acaa">properties</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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">test</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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">type</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"string"</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 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 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 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 module" style="color:#00009f">export</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">default</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 parameter">source</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">const</span><span class="token plain"> options </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">getOptions</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">this</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" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function" style="color:#d73a49">validate</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">schema</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> options</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 literal-property property" style="color:#36acaa">name</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Example Loader"</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 literal-property property" style="color:#36acaa">baseDataPath</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"options"</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 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 comment" style="color:#999988;font-style:italic">// Apply some transformations to the source...</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 control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string string" style="color:#e3116c">export default </span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation known-class-name class-name">JSON</span><span class="token template-string interpolation punctuation" style="color:#393A34">.</span><span class="token template-string interpolation method function property-access" style="color:#d73a49">stringify</span><span class="token template-string interpolation punctuation" style="color:#393A34">(</span><span class="token template-string interpolation">source</span><span class="token template-string interpolation punctuation" style="color:#393A34">)</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string template-punctuation string" style="color:#e3116c">`</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><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>loader-utils 主要有以下工具方法：</p><ul><li><code>parseQuery</code>：解析 loader 的 query 参数，返回一个对象。</li><li><code>stringifyRequest</code>：将请求的资源转换为可以在 loader 生成的代码中 require 或 import 使用的相对路径字符串，同时避免绝对路径导致重新计算 hash 值。<div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">loaderUtils</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">stringifyRequest</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">this</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"./test.js"</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 comment" style="color:#999988;font-style:italic">// "\"./test.js\""</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></li><li><code>urlToRequest</code>：将请求的资源路径转换成 webpack 可以处理的形式。<div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> url </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"~path/to/module.js"</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">const</span><span class="token plain"> request </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> loaderUtils</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">urlToRequest</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">url</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">// "path/to/module.js"</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></li><li><code>interpolateName</code>：对文件名模板进行插值。<div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic">// loaderContext.resourcePath = "/absolute/path/to/app/js/hzfe.js"</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">loaderUtils</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">interpolateName</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">loaderContext</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"js/[hash].script.[ext]"</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 class="token literal-property property" style="color:#36acaa">content</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token spread operator" 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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic">// => js/9473fdd0d880a43c21b7778d34872157.script.js</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></li><li><code>getHashDigest</code>：获取文件内容的 hash 值。</li></ul><p>在编写 loader 的过程中，还可以利用 loaderContext 对象来获取 loader 的相关信息和进行一些高级的操作，常见的属性和方法有：</p><ul><li><code>this.addDependency</code>：加入一个文件，作为 loader 产生的结果的依赖，使其在有任何变化时可以被监听到，从而触发重新编译。</li><li><code>this.async</code>：告诉 loader-runner 这个 loader 将会异步的执行回调。</li><li><code>this.cacheable</code>：默认情况下，将 loader 的处理结果标记为可缓存。传入 false 可以关闭 loader 处理结果的缓存能力。</li><li><code>this.fs</code>：用于访问 compilation 的 inputFileSystem 属性。</li><li><code>this.getOptions</code>：提取 loader 的配置选项。从 webpack 5 开始，可以获取到 loader 上下文对象，用于替代 <code>loader-utils</code> 中的 getOptions 方法。</li><li><code>this.mode</code>： webpack 的运行模式，可以是 "development" 或 "production"。</li><li><code>this.query</code>：如果 loader 配置了 options 对象，则指向这个对象。如果 loader 没有 options，而是以 query 字符串作为参数，query 则是一个以 <code>?</code> 开头的字符串。</li></ul><p>以上内容是编写一个 loader 的关键点，想要学习更详细的关于编写 loader 的指导，可以参考官方的 <a href="https://webpack.js.org/contribute/writing-a-loader/#guidelines" target="_blank" rel="noopener noreferrer">guidelines</a>。</p><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="2-webpack-loader-工作机制">2. webpack loader 工作机制<a href="#2-webpack-loader-工作机制" class="hash-link" aria-label="Direct link to 2. webpack loader 工作机制" title="Direct link to 2. webpack loader 工作机制"></a></h3><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="21-根据-modulerules-解析-loader-加载规则">2.1 根据 module.rules 解析 loader 加载规则<a href="#21-根据-modulerules-解析-loader-加载规则" class="hash-link" aria-label="Direct link to 2.1 根据 module.rules 解析 loader 加载规则" title="Direct link to 2.1 根据 module.rules 解析 loader 加载规则"></a></h4><p>当 webpack 处理一个模块（module）时，会根据配置文件中 <code>module.rules</code> 的规则，使用 loader 处理对应资源，得到可供 webpack 使用的 JavaScript 模块。</p><p>根据具体的配置情况，loader 会有不同的类型，可以影响 loader 的执行顺序。具体类型如下所示：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token literal-property property" style="color:#36acaa">rules</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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// pre 前置 loader</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><span class="token literal-property property" style="color:#36acaa">enforce</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"pre"</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">test</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-source language-regex" style="color:#36acaa">\.js$</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">loader</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"eslint-loader"</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// normal loader</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">test</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-source language-regex" style="color:#36acaa">\.js$</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">loader</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"babel-loader"</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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// post 后置 loader</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><span class="token literal-property property" style="color:#36acaa">enforce</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"post"</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">test</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-source language-regex" style="color:#36acaa">\.js$</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">loader</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"eslint-loader"</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><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><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>以及内联使用的 inline loader：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"style-loader!css-loader!sass-loader!./hzfe.scss"</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>在正常的执行流程中，这些不同类型的 loader 的执行顺序是：<code>pre -> normal -> inline -> post</code>。在下一节将会提到的 pitch 流程中，这些 loader 的执行顺序是反过来的：<code>post -> inline -> normal -> pre</code>。</p><p>对于内联 loader，可以通知修饰前缀改变 loader 的执行顺序：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic">// ! 前缀会禁用 normal loader</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token constant" style="color:#36acaa">HZFE</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"!./hzfe.js"</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 comment" style="color:#999988;font-style:italic">// -! 前缀会禁用 pre loader 和 normal loader</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token constant" style="color:#36acaa">HZFE</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"-!./hzfe.js"</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 comment" style="color:#999988;font-style:italic">// !! 前缀会禁用 pre、normal 和 post loader</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> </span><span class="token constant" style="color:#36acaa">HZFE</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"!!./hzfe.js"</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>一般情况下，<code>!</code> 前缀和 inline loader 一起使用仅出现在 loader（如 style-loader）生成的代码中，webpack 官方不建议用户同时使用 inline loader 和 <code>!</code> 前缀。</p><p>webpack rules 中配置的 loader 可以是多个链式串联的。在正常流程中，链式 loader 会按照从后往前的顺序执行。</p><ul><li>最后的 loader 最先执行，它接收的是资源文件（resource file）的内容。</li><li>第一个 loader 最后执行，它将返回 JavaScript 模块和可选的 source map。</li><li>位于中间的 loader，对接收和返回没有特定要求，只要能处理之前 loader 返回的内容，产出下一个 loader 能够理解的内容就可以。</li></ul><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="22-loader-runner-的执行流程">2.2 loader-runner 的执行流程<a href="#22-loader-runner-的执行流程" class="hash-link" aria-label="Direct link to 2.2 loader-runner 的执行流程" title="Direct link to 2.2 loader-runner 的执行流程"></a></h4><p>webpack 调用 loader 的时机在触发 compilation 的 buildModule 钩子之后。webpack 会在 <code>NormalModule.js</code> 中，调用 runLoaders 运行 loader：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token function" style="color:#d73a49">runLoaders</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 literal-property property" style="color:#36acaa">resource</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">this</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">resource</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 资源文件的路径，可以有查询字符串。如：'./test.txt?query'</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">loaders</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">this</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">loaders</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// loader 的路径。</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">context</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> loaderContext</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 传递给 loader 的上下文</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function-variable function" style="color:#d73a49">processResource</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 parameter">loaderContext</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> resourcePath</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> callback</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" 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 comment" style="color:#999988;font-style:italic">// 获取资源的方式，有 scheme 的文件通过 readResourceForScheme 读取，否则通过 fs.readFile 读取。</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">const</span><span class="token plain"> resource </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> loaderContext</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">resource</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">const</span><span class="token plain"> scheme </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">getScheme</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">resource</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 keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">scheme</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"> hooks</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">readResourceForScheme</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 control-flow" style="color:#00009f">for</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">scheme</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 method function property-access" style="color:#d73a49">callAsync</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">resource</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">this</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 parameter">err</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> result</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" 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 comment" style="color:#999988;font-style:italic">// ...</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"> </span><span class="token function" style="color:#d73a49">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> result</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 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><span class="token keyword control-flow" style="color:#00009f">else</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"> loaderContext</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">addDependency</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">resourcePath</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"> fs</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">readFile</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">resourcePath</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> callback</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 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 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 parameter">err</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> result</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" 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 comment" style="color:#999988;font-style:italic">// 当 loader 转换完成后，会将结果返回到 webpack 中继续处理。</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">processResult</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">err</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> result</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">result</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 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>runLoaders 函数来自 <code>loader-runner</code> 包。在介绍 runLoaders 的具体流程之前，先介绍一下 pitch 阶段，上一节中所讲的这种从后往前执行 loader 的流程，一般叫做 normal 阶段。与之相对的，还有一种叫做 pitch 阶段的流程。</p><p>一个 loader 如果在导出的函数的 pitch 属性上挂在了方法，那这个方法将在 pitch 阶段执行。pitch 阶段不同于 normal 阶段，pitch 阶段的执行顺序是从前往后的，整个流程类似浏览器事件模型或洋葱模型，pitch 阶段先从前往后执行 loader，然后再进入 normal 阶段从后往前执行 loader。注意，pitch 阶段一般不返回值，一旦 pitch 阶段有 loader 返回值，则从这里开始进入从后往前执行的 normal 阶段。</p><p>loader-runner 的具体流程如下：</p><ol><li><p>处理从 webpack 接收的 context，继续添加必要的属性和辅助方法。</p></li><li><p>iteratePitchingLoaders 处理 pitch loader。</p><p>如果我们给一个 module 配置了三个 loader，每个 loader 都配置了 pitch 函数：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">module</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">exports</span><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><br></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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">module</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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">rules</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><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 comment" style="color:#999988;font-style:italic">//...</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">use</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">"a-loader"</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"b-loader"</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"c-loader"</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 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 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 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><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>那么处理这个 module 的流程如下：</p><div class="language-shell codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-shell codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token operator" style="color:#393A34">|</span><span class="token plain">- a-loader </span><span class="token variable" style="color:#36acaa">`</span><span class="token variable" style="color:#36acaa">pitch</span><span class="token variable" style="color:#36acaa">`</span><span class="token plain"></span><br></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">- b-loader </span><span class="token variable" style="color:#36acaa">`</span><span class="token variable" style="color:#36acaa">pitch</span><span class="token variable" style="color:#36acaa">`</span><span class="token plain"></span><br></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">- c-loader </span><span class="token variable" style="color:#36acaa">`</span><span class="token variable" style="color:#36acaa">pitch</span><span class="token variable" style="color:#36acaa">`</span><span class="token plain"></span><br></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">- requested module is picked up as a dependency</span><br></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">- c-loader normal execution</span><br></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">- b-loader normal execution</span><br></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">- a-loader normal execution</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>如果 b-loader 在 pitch 中提前返回了值，那么流程如下：</p><div class="language-shell codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-shell codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token operator" style="color:#393A34">|</span><span class="token plain">- a-loader </span><span class="token variable" style="color:#36acaa">`</span><span class="token variable" style="color:#36acaa">pitch</span><span class="token variable" style="color:#36acaa">`</span><span class="token plain"></span><br></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">- b-loader </span><span class="token variable" style="color:#36acaa">`</span><span class="token variable" style="color:#36acaa">pitch</span><span class="token variable" style="color:#36acaa">`</span><span class="token plain"> returns a module</span><br></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">- a-loader normal execution</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></li><li><p>iterateNormalLoaders 处理 normal loader。</p><p>当 pitch loader 的流程处理完后，就来到了处理 normal loader 的流程。处理 normal loader 的流程和 pitch loader 相似，只是从后往前迭代。</p><p>iterateNormalLoaders 和 iteratePitchingLoaders 都会调用 runSyncOrAsync 来执行 loader。runSyncOrAsync 会提供 context.async，这是一个返回 callback 的 async 函数，用于异步处理。</p></li></ol><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="3-常见-webpack-loader-原理解析">3. 常见 webpack loader 原理解析<a href="#3-常见-webpack-loader-原理解析" class="hash-link" aria-label="Direct link to 3. 常见 webpack loader 原理解析" title="Direct link to 3. 常见 webpack loader 原理解析"></a></h3><p>loader 本身的操作并不复杂，就是一个负责转换其他资源到 JavaScript 模块的函数。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="31-raw-loader-分析">3.1 raw-loader 分析<a href="#31-raw-loader-分析" class="hash-link" aria-label="Direct link to 3.1 raw-loader 分析" title="Direct link to 3.1 raw-loader 分析"></a></h4><p>该 loader 是功能非常简单的同步 loader，它的核心步骤是从文件原始内容中取得序列化的字符串，修复 JSON 序列化特殊字符时的 bug，添加导出语句，使其成为 JavaScript 模块。</p><p>该 loader 在 webpack 5 中已废弃，直接使用 asset modules 的功能代替即可。该 loader 源码如下：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports punctuation" style="color:#393A34">{</span><span class="token imports"> getOptions </span><span class="token imports punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"loader-utils"</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 module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports punctuation" style="color:#393A34">{</span><span class="token imports"> validate </span><span class="token imports punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"schema-utils"</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 module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports">schema</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"./options.json"</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 module" style="color:#00009f">export</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">default</span><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">rawLoader</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">source</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">const</span><span class="token plain"> options </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">getOptions</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">this</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" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function" style="color:#d73a49">validate</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">schema</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> options</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 literal-property property" style="color:#36acaa">name</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Raw Loader"</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 literal-property property" style="color:#36acaa">baseDataPath</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"options"</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 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">const</span><span class="token plain"> json </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token known-class-name class-name">JSON</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">stringify</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">source</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 method function property-access" style="color:#d73a49">replace</span><span class="token punctuation" style="color:#393A34">(</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-source language-regex" style="color:#36acaa">\u2028</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-flags" style="color:#36acaa">g</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"\\u2028"</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 method function property-access" style="color:#d73a49">replace</span><span class="token punctuation" style="color:#393A34">(</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-source language-regex" style="color:#36acaa">\u2029</span><span class="token regex regex-delimiter" style="color:#36acaa">/</span><span class="token regex regex-flags" style="color:#36acaa">g</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"\\u2029"</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" 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">const</span><span class="token plain"> esModule </span><span class="token operator" 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">typeof</span><span class="token plain"> options</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">esModule</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">!==</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"undefined"</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">?</span><span class="token plain"> options</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">esModule</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token boolean" style="color:#36acaa">true</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 control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation">esModule </span><span class="token template-string interpolation operator" style="color:#393A34">?</span><span class="token template-string interpolation"> </span><span class="token template-string interpolation string" style="color:#e3116c">"export default"</span><span class="token template-string interpolation"> </span><span class="token template-string interpolation operator" style="color:#393A34">:</span><span class="token template-string interpolation"> </span><span class="token template-string interpolation string" style="color:#e3116c">"module.exports ="</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string string" style="color:#e3116c"> </span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation">json</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string string" style="color:#e3116c">;</span><span class="token template-string template-punctuation string" style="color:#e3116c">`</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><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><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="32-babel-loader-分析">3.2 babel-loader 分析<a href="#32-babel-loader-分析" class="hash-link" aria-label="Direct link to 3.2 babel-loader 分析" title="Direct link to 3.2 babel-loader 分析"></a></h4><p>babel loader 是一个综合了同步和异步的 loader，在使用缓存配置时以异步模式运行，否则以同步方式运行。该 loader 的主要源码如下：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token comment" style="color:#999988;font-style:italic">// imports ...</span><span class="token plain"></span><br></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><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">const</span><span class="token plain"> </span><span class="token function-variable function" style="color:#d73a49">transpile</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 parameter">source</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> options</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 comment" style="color:#999988;font-style:italic">// ...</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">let</span><span class="token plain"> result</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">try</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"> result </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> babel</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">transform</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">source</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> options</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><span class="token keyword control-flow" style="color:#00009f">catch</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">error</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 comment" style="color:#999988;font-style:italic">// ...</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 comment" style="color:#999988;font-style:italic">// ...</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 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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">code</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> code</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 literal-property property" style="color:#36acaa">map</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> map</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 literal-property property" style="color:#36acaa">metadata</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> metadata</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 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 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 comment" style="color:#999988;font-style:italic">// ...</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">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 parameter">source</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> inputSourceMap</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 comment" style="color:#999988;font-style:italic">// ...</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 control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">cacheDirectory</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">const</span><span class="token plain"> callback </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">this</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">async</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 keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">cache</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 literal-property property" style="color:#36acaa">directory</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> cacheDirectory</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 literal-property property" style="color:#36acaa">identifier</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> cacheIdentifier</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 literal-property property" style="color:#36acaa">source</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> source</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 literal-property property" style="color:#36acaa">options</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> options</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 literal-property property" style="color:#36acaa">transform</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> transpile</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 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 parameter">err</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> </span><span class="token parameter punctuation" style="color:#393A34">{</span><span class="token parameter"> code</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> map</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> metadata </span><span class="token parameter punctuation" style="color:#393A34">}</span><span class="token parameter"> </span><span class="token parameter operator" style="color:#393A34">=</span><span class="token parameter"> </span><span class="token parameter punctuation" style="color:#393A34">{</span><span class="token parameter punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" 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 control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">err</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">err</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" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> metadataSubscribers</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">s</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token function" style="color:#d73a49">passMetadata</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">s</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">this</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> metadata</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" style="display:inline-block"></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"> </span><span class="token function" style="color:#d73a49">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> code</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> map</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 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" 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">const</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"> code</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> map</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> metadata </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">transpile</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">source</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> options</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" 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">this</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">callback</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> code</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> map</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><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>babel-loader 通过 callback 传递了经过 babel.transform 转换后的代码及 source map。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="33-style-loader-与-css-loader-分析">3.3 style-loader 与 css-loader 分析<a href="#33-style-loader-与-css-loader-分析" class="hash-link" aria-label="Direct link to 3.3 style-loader 与 css-loader 分析" title="Direct link to 3.3 style-loader 与 css-loader 分析"></a></h4><p>style-loader 负责将样式插入到 DOM 中，使样式对页面生效。css-loader 主要负责处理 import、url 路径等外部引用。</p><p>style-loader 只有 pitch 函数。css-loader 是 normal module。整个执行流程是先执行 style-loader 阶段，style-loader 会创建形如 <code>require(!!./hzfe.css)</code> 的代码返回给 webpack。webpack 会再次调用 css-loader 处理样式，css-loader 会返回包含 runtime 的 js 模块给 webpack 去解析。style-loader 在上一步注入 <code>require(!!./hzfe.css)</code> 的同时，也注入了添加 style 标签的代码。这样，在运行时（浏览器中），style-loader 就可以把 css-loader 的样式插入到页面中。</p><p>常见的疑问就是为什么不按照 normal 模式组织 style-loader 和 css-loader。</p><p>首先 css-loader 返回的是形如这样的代码：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">import</span><span class="token plain"> </span><span class="token imports">___CSS_LOADER_API_IMPORT___</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">from</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"../node_modules/_css-loader@5.1.3@css-loader/dist/runtime/api.js"</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"> ___CSS_LOADER_EXPORT___ </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">___CSS_LOADER_API_IMPORT___</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 punctuation" style="color:#393A34">(</span><span class="token parameter">i</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 control-flow" style="color:#00009f">return</span><span class="token plain"> i</span><span class="token punctuation" style="color:#393A34">[</span><span class="token number" style="color:#36acaa">1</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 plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic">// Module</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">___CSS_LOADER_EXPORT___</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">push</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"> module</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">id</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 string" style="color:#e3116c">".hzfe{\r\n height: 100px;\r\n}"</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 string" style="color:#e3116c">""</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 plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic">// Exports</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword module" style="color:#00009f">export</span><span class="token plain"> </span><span class="token keyword module" style="color:#00009f">default</span><span class="token plain"> ___CSS_LOADER_EXPORT___</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>style-loader 无法在编译时获取 CSS 相关的内容，因为 style-loader 无法处理 css-loader 生成结果的 runtime 依赖。style-loader 也无法在运行时获取 CSS 相关的内容，因为无论怎样拼接运行时代码，都无法获取到 CSS 的内容。</p><p>作为替代，style-loader 采用了 pitch 方案，style-loader 的核心功能如下所示：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">style-loader</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">module</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">exports</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method-variable function-variable method function property-access" style="color:#d73a49">pitch</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 parameter">request</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"> result </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><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 生成 require CSS 文件的语句，交给 css-loader 解析 得到包含 CSS 内容的 JS 模块</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 其中 !! 是为了避免 webpack 解析时递归调用 style-loader</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string string" style="color:#e3116c">var content=require("</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation">loaderUtils</span><span class="token template-string interpolation punctuation" style="color:#393A34">.</span><span class="token template-string interpolation method function property-access" style="color:#d73a49">stringifyRequest</span><span class="token template-string interpolation punctuation" style="color:#393A34">(</span><span class="token template-string interpolation keyword" style="color:#00009f">this</span><span class="token template-string interpolation punctuation" style="color:#393A34">,</span><span class="token template-string interpolation"> </span><span class="token template-string interpolation template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string interpolation template-string string" style="color:#e3116c">!!</span><span class="token template-string interpolation template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation template-string interpolation">request</span><span class="token template-string interpolation template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string interpolation template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string interpolation punctuation" style="color:#393A34">)</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string string" style="color:#e3116c">")</span><span class="token template-string template-punctuation string" style="color:#e3116c">`</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 在运行时调用 addStyle 把 CSS 内容插入到 DOM 中</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string string" style="color:#e3116c">require("</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation">loaderUtils</span><span class="token template-string interpolation punctuation" style="color:#393A34">.</span><span class="token template-string interpolation method function property-access" style="color:#d73a49">stringifyRequest</span><span class="token template-string interpolation punctuation" style="color:#393A34">(</span><span class="token template-string interpolation keyword" style="color:#00009f">this</span><span class="token template-string interpolation punctuation" style="color:#393A34">,</span><span class="token template-string interpolation"> </span><span class="token template-string interpolation template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string interpolation template-string string" style="color:#e3116c">!</span><span class="token template-string interpolation template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">${</span><span class="token template-string interpolation template-string interpolation">path</span><span class="token template-string interpolation template-string interpolation punctuation" style="color:#393A34">.</span><span class="token template-string interpolation template-string interpolation method function property-access" style="color:#d73a49">join</span><span class="token template-string interpolation template-string interpolation punctuation" style="color:#393A34">(</span><span class="token template-string interpolation template-string interpolation">__dirname</span><span class="token template-string interpolation template-string interpolation punctuation" style="color:#393A34">,</span><span class="token template-string interpolation template-string interpolation"> </span><span class="token template-string interpolation template-string interpolation string" style="color:#e3116c">"add-style.js"</span><span class="token template-string interpolation template-string interpolation punctuation" style="color:#393A34">)</span><span class="token template-string interpolation template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string interpolation template-string template-punctuation string" style="color:#e3116c">`</span><span class="token template-string interpolation punctuation" style="color:#393A34">)</span><span class="token template-string interpolation interpolation-punctuation punctuation" style="color:#393A34">}</span><span class="token template-string string" style="color:#e3116c">")(content)</span><span class="token template-string template-punctuation string" style="color:#e3116c">`</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 如果发现启用了 CSS modules，则默认导出它</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token string" style="color:#e3116c">"if(content.locals) module.exports = content.locals"</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 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"> result</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">join</span><span class="token punctuation" style="color:#393A34">(</span><span class="token string" style="color:#e3116c">";"</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><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><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">add-style.js</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><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 parameter">content</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"> style </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token dom variable" style="color:#36acaa">document</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">createElement</span><span class="token punctuation" style="color:#393A34">(</span><span class="token string" style="color:#e3116c">"style"</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"> style</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">innerHTML</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> content</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 dom variable" style="color:#36acaa">document</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">head</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">appendChild</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">style</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><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 pitch 阶段，style-loader 生成 require CSS 以及注入 runtime 的代码。该结果会返回给 webpack 进一步解析，css-loader 返回的结果会作为模块在运行时导入，在运行时能够获得 CSS 的内容，然后调用 add-style.js 把 CSS 内容插入到 DOM 中。</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://webpack.js.org/contribute/writing-a-loader/" target="_blank" rel="noopener noreferrer">writting a loader</a></li><li><a href="https://webpack.js.org/api/loaders/" target="_blank" rel="noopener noreferrer">Loader Interface</a></li><li><a href="https://github.com/webpack/loader-runner" target="_blank" rel="noopener noreferrer">loader runner</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/book3/browser-memory-leaks"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">浏览器：如何定位内存泄露</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/awesome-interview/book3/frame-react-hooks"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">框架：React Hooks 实现原理</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-编写-webpack-loader" class="table-of-contents__link toc-highlight">1. 编写 webpack loader</a><ul><li><a href="#11-同步-loader" class="table-of-contents__link toc-highlight">1.1 同步 loader</a></li><li><a href="#12-异步-loader" class="table-of-contents__link toc-highlight">1.2 异步 loader</a></li><li><a href="#13-loader-开发辅助工具及-loadercontext" class="table-of-contents__link toc-highlight">1.3 loader 开发辅助工具及 loaderContext</a></li></ul></li><li><a href="#2-webpack-loader-工作机制" class="table-of-contents__link toc-highlight">2. webpack loader 工作机制</a><ul><li><a href="#21-根据-modulerules-解析-loader-加载规则" class="table-of-contents__link toc-highlight">2.1 根据 module.rules 解析 loader 加载规则</a></li><li><a href="#22-loader-runner-的执行流程" class="table-of-contents__link toc-highlight">2.2 loader-runner 的执行流程</a></li></ul></li><li><a href="#3-常见-webpack-loader-原理解析" class="table-of-contents__link toc-highlight">3. 常见 webpack loader 原理解析</a><ul><li><a href="#31-raw-loader-分析" class="table-of-contents__link toc-highlight">3.1 raw-loader 分析</a></li><li><a href="#32-babel-loader-分析" class="table-of-contents__link toc-highlight">3.2 babel-loader 分析</a></li><li><a href="#33-style-loader-与-css-loader-分析" class="table-of-contents__link toc-highlight">3.3 style-loader 与 css-loader 分析</a></li></ul></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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