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- 1-5 使用 Plugin
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- 1-7 核心概念
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- 2-4 Resolve
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- 2-5 Plugins
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- 2-6 DevServer
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- 2-7 其它配置项
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- 2-8 整体配置结构
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- 2-9 多种配置类型
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- 2-10 配置总结
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- 3-1 使用 ES6 语言
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- 3-2 使用 TypeScript 语言
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- 3-3 使用 Flow 检查器
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- 3-4 使用 SCSS
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- 3-5 使用 PostCSS
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- 3-6 使用 React 框架
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- 3-7 使用 Vue 框架
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- 3-8 使用 Angular2 框架
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- 3-9 为单页应用生成 HTML
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- 3-10 管理多个单页应用
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- 3-11 构建同构应用
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- 3-12 构建 Electron 应用
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- 3-13 构建 Npm 模块
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- 3-14 构建离线应用
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- 3-15 搭配 Npm Script
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- 3-16 检查代码
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- 3-17 通过 Node.js API 启动 Webpack
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- <li class="chapter " data-level="1.5.18" data-path="../3实战/3-18使用WebpackDevMiddleware.html">
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- 3-18 使用 Webpack Dev Middleware
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- 3-19 加载图片
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- 3-20 加载SVG
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- 3-21 加载 Source Map
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- 3-22 实战总结
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- <li class="chapter " data-level="1.6" data-path="../4优化/">
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- <a href="../4优化/">
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- 第4章 优化
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- <li class="chapter " data-level="1.6.1" data-path="../4优化/4-1缩小文件搜索范围.html">
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- <a href="../4优化/4-1缩小文件搜索范围.html">
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- 4-1 缩小文件搜索范围
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- <li class="chapter " data-level="1.6.2" data-path="../4优化/4-2使用DllPlugin.html">
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- 4-2 使用 DllPlugin
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- <li class="chapter " data-level="1.6.3" data-path="../4优化/4-3使用HappyPack.html">
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- <a href="../4优化/4-3使用HappyPack.html">
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- 4-3 使用 HappyPack
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- 4-4 使用 ParallelUglifyPlugin
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- 4-5 使用自动刷新
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- <a href="../4优化/4-6开启模块热替换.html">
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- <h1 id="5-3-编写-loader">5-3 编写 Loader</h1>
- <p>Loader 就像是一个翻译员,能把源文件经过转化后输出新的结果,并且一个文件还可以链式的经过多个翻译员翻译。</p>
- <p>以处理 SCSS 文件为例:</p>
- <ol>
- <li>SCSS 源代码会先交给 sass-loader 把 SCSS 转换成 CSS;</li>
- <li>把 sass-loader 输出的 CSS 交给 css-loader 处理,找出 CSS 中依赖的资源、压缩 CSS 等;</li>
- <li>把 css-loader 输出的 CSS 交给 style-loader 处理,转换成通过脚本加载的 JavaScript 代码;</li>
- </ol>
- <p>可以看出以上的处理过程需要有顺序的链式执行,先 sass-loader 再 css-loader 再 style-loader。
- 以上处理的 Webpack 相关配置如下:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = {
- <span class="hljs-built_in">module</span>: {
- rules: [
- {
- <span class="hljs-comment">// 增加对 SCSS 文件的支持</span>
- test: <span class="hljs-regexp">/\.scss$/</span>,
- <span class="hljs-comment">// SCSS 文件的处理顺序为先 sass-loader 再 css-loader 再 style-loader</span>
- use: [
- <span class="hljs-string">'style-loader'</span>,
- {
- loader:<span class="hljs-string">'css-loader'</span>,
- <span class="hljs-comment">// 给 css-loader 传入配置项</span>
- options:{
- minimize:<span class="hljs-literal">true</span>,
- }
- },
- <span class="hljs-string">'sass-loader'</span>],
- },
- ]
- },
- };
- </code></pre>
- <h2 id="loader-的职责">Loader 的职责</h2>
- <p>由上面的例子可以看出:一个 Loader 的职责是单一的,只需要完成一种转换。
- 如果一个源文件需要经历多步转换才能正常使用,就通过多个 Loader 去转换。
- 在调用多个 Loader 去转换一个文件时,每个 Loader 会链式的顺序执行,
- 第一个 Loader 将会拿到需处理的原内容,上一个 Loader 处理后的结果会传给下一个接着处理,最后的 Loader 将处理后的最终结果返回给 Webpack。</p>
- <p>所以,在你开发一个 Loader 时,请保持其职责的单一性,你只需关心输入和输出。</p>
- <h2 id="loader-基础">Loader 基础</h2>
- <p>由于 Webpack 是运行在 Node.js 之上的,一个 Loader 其实就是一个 Node.js 模块,这个模块需要导出一个函数。
- 这个导出的函数的工作就是获得处理前的原内容,对原内容执行处理后,返回处理后的内容。</p>
- <p>一个最简单的 Loader 的源码如下:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// source 为 compiler 传递给 Loader 的一个文件的原内容</span>
- <span class="hljs-comment">// 该函数需要返回处理后的内容,这里简单起见,直接把原内容返回了,相当于该 Loader 没有做任何转换</span>
- <span class="hljs-keyword">return</span> source;
- };
- </code></pre>
- <p>由于 Loader 运行在 Node.js 中,你可以调用任何 Node.js 自带的 API,或者安装第三方模块进行调用:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">const</span> sass = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node-sass'</span>);
- <span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-keyword">return</span> sass(source);
- };
- </code></pre>
- <h2 id="loader-进阶">Loader 进阶</h2>
- <p>以上只是个最简单的 Loader,Webpack 还提供一些 API 供 Loader 调用,下面来一一介绍。</p>
- <h3 id="获得-loader-的-options">获得 Loader 的 options</h3>
- <p>在最上面处理 SCSS 文件的 Webpack 配置中,给 css-loader 传了 options 参数,以控制 css-loader。
- 如何在自己编写的 Loader 中获取到用户传入的 options 呢?需要这样做:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">const</span> loaderUtils = <span class="hljs-built_in">require</span>(<span class="hljs-string">'loader-utils'</span>);
- <span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// 获取到用户给当前 Loader 传入的 options</span>
- <span class="hljs-keyword">const</span> options = loaderUtils.getOptions(<span class="hljs-keyword">this</span>);
- <span class="hljs-keyword">return</span> source;
- };
- </code></pre>
- <h3 id="返回其它结果">返回其它结果</h3>
- <p>上面的 Loader 都只是返回了原内容转换后的内容,但有些场景下还需要返回除了内容之外的东西。</p>
- <p>例如以用 babel-loader 转换 ES6 代码为例,它还需要输出转换后的 ES5 代码对应的 Source Map,以方便调试源码。
- 为了把 Source Map 也一起随着 ES5 代码返回给 Webpack,可以这样写:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// 通过 this.callback 告诉 Webpack 返回的结果</span>
- <span class="hljs-keyword">this</span>.callback(<span class="hljs-literal">null</span>, source, sourceMaps);
- <span class="hljs-comment">// 当你使用 this.callback 返回内容时,该 Loader 必须返回 undefined,</span>
- <span class="hljs-comment">// 以让 Webpack 知道该 Loader 返回的结果在 this.callback 中,而不是 return 中 </span>
- <span class="hljs-keyword">return</span>;
- };
- </code></pre>
- <p>其中的 <code>this.callback</code> 是 Webpack 给 Loader 注入的 API,以方便 Loader 和 Webpack 之间通信。
- <code>this.callback</code> 的详细使用方法如下:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">this</span>.callback(
- <span class="hljs-comment">// 当无法转换原内容时,给 Webpack 返回一个 Error</span>
- err: <span class="hljs-built_in">Error</span> | <span class="hljs-literal">null</span>,
- <span class="hljs-comment">// 原内容转换后的内容</span>
- content: string | Buffer,
- <span class="hljs-comment">// 用于把转换后的内容得出原内容的 Source Map,方便调试</span>
- sourceMap?: SourceMap,
- <span class="hljs-comment">// 如果本次转换为原内容生成了 AST 语法树,可以把这个 AST 返回,</span>
- <span class="hljs-comment">// 以方便之后需要 AST 的 Loader 复用该 AST,以避免重复生成 AST,提升性能</span>
- abstractSyntaxTree?: AST
- );
- </code></pre>
- <blockquote>
- <p>Source Map 的生成很耗时,通常在开发环境下才会生成 Source Map,其它环境下不用生成,以加速构建。
- 为此 Webpack 为 Loader 提供了 <code>this.sourceMap</code> API 去告诉 Loader 当前构建环境下用户是否需要 Source Map。
- 如果你编写的 Loader 会生成 Source Map,请考虑到这点。</p>
- </blockquote>
- <h3 id="同步与异步">同步与异步</h3>
- <p>Loader 有同步和异步之分,上面介绍的 Loader 都是同步的 Loader,因为它们的转换流程都是同步的,转换完成后再返回结果。
- 但在有些场景下转换的步骤只能是异步完成的,例如你需要通过网络请求才能得出结果,如果采用同步的方式网络请求就会阻塞整个构建,导致构建非常缓慢。</p>
- <p>在转换步骤是异步时,你可以这样:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// 告诉 Webpack 本次转换是异步的,Loader 会在 callback 中回调结果</span>
- <span class="hljs-keyword">var</span> callback = <span class="hljs-keyword">this</span>.async();
- someAsyncOperation(source, <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">err, result, sourceMaps, ast</span>) </span>{
- <span class="hljs-comment">// 通过 callback 返回异步执行后的结果</span>
- callback(err, result, sourceMaps, ast);
- });
- };
- </code></pre>
- <h3 id="处理二进制数据">处理二进制数据</h3>
- <p>在默认的情况下,Webpack 传给 Loader 的原内容都是 UTF-8 格式编码的字符串。
- 但有些场景下 Loader 不是处理文本文件,而是处理二进制文件,例如 file-loader,就需要 Webpack 给 Loader 传入二进制格式的数据。
- 为此,你需要这样编写 Loader:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// 在 exports.raw === true 时,Webpack 传给 Loader 的 source 是 Buffer 类型的</span>
- source <span class="hljs-keyword">instanceof</span> Buffer === <span class="hljs-literal">true</span>;
- <span class="hljs-comment">// Loader 返回的类型也可以是 Buffer 类型的</span>
- <span class="hljs-comment">// 在 exports.raw !== true 时,Loader 也可以返回 Buffer 类型的结果</span>
- <span class="hljs-keyword">return</span> source;
- };
- <span class="hljs-comment">// 通过 exports.raw 属性告诉 Webpack 该 Loader 是否需要二进制数据 </span>
- <span class="hljs-built_in">module</span>.exports.raw = <span class="hljs-literal">true</span>;
- </code></pre>
- <p>以上代码中最关键的代码是最后一行 <code>module.exports.raw = true;</code>,没有该行 Loader 只能拿到字符串。</p>
- <h3 id="缓存加速">缓存加速</h3>
- <p>在有些情况下,有些转换操作需要大量计算非常耗时,如果每次构建都重新执行重复的转换操作,构建将会变得非常缓慢。
- 为此,Webpack 会默认缓存所有 Loader 的处理结果,也就是说在需要被处理的文件或者其依赖的文件没有发生变化时,
- 是不会重新调用对应的 Loader 去执行转换操作的。</p>
- <p>如果你想让 Webpack 不缓存该 Loader 的处理结果,可以这样:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// 关闭该 Loader 的缓存功能</span>
- <span class="hljs-keyword">this</span>.cacheable(<span class="hljs-literal">false</span>);
- <span class="hljs-keyword">return</span> source;
- };
- </code></pre>
- <h2 id="其它-loader-api">其它 Loader API</h2>
- <p>除了以上提到的在 Loader 中能调用的 Webpack API 外,还存在以下常用 API:</p>
- <ul>
- <li><p><code>this.context</code>:当前处理文件的所在目录,假如当前 Loader 处理的文件是 <code>/src/main.js</code>,则 <code>this.context</code> 就等于 <code>/src</code>。</p>
- </li>
- <li><p><code>this.resource</code>:当前处理文件的完整请求路径,包括 querystring,例如 <code>/src/main.js?name=1</code>。</p>
- </li>
- <li><p><code>this.resourcePath</code>:当前处理文件的路径,例如 <code>/src/main.js</code>。</p>
- </li>
- <li><p><code>this.resourceQuery</code>:当前处理文件的 querystring。</p>
- </li>
- <li><p><code>this.target</code>:等于 Webpack 配置中的 Target,详情见 <a href="../2配置/2-7其它配置项.html#Target">2-7其它配置项-Target</a>。</p>
- </li>
- <li><p><code>this.loadModule</code>:当 Loader 在处理一个文件时,如果依赖其它文件的处理结果才能得出当前文件的结果时,
- 就可以通过 <code>this.loadModule(request: string, callback: function(err, source, sourceMap, module))</code> 去获得 <code>request</code> 对应文件的处理结果。</p>
- </li>
- <li><p><code>this.resolve</code>:像 <code>require</code> 语句一样获得指定文件的完整路径,使用方法为 <code>resolve(context: string, request: string, callback: function(err, result: string))</code>。</p>
- </li>
- <li><p><code>this.addDependency</code>:给当前处理文件添加其依赖的文件,以便再其依赖的文件发生变化时,会重新调用 Loader 处理该文件。使用方法为 <code>addDependency(file: string)</code>。</p>
- </li>
- <li><p><code>this.addContextDependency</code>:和 <code>addDependency</code> 类似,但 <code>addContextDependency</code> 是把整个目录加入到当前正在处理文件的依赖中。使用方法为 <code>addContextDependency(directory: string)</code>。</p>
- </li>
- <li><p><code>this.clearDependencies</code>:清除当前正在处理文件的所有依赖,使用方法为 <code>clearDependencies()</code>。</p>
- </li>
- <li><p><code>this.emitFile</code>:输出一个文件,使用方法为 <code>emitFile(name: string, content: Buffer|string, sourceMap: {...})</code>。</p>
- </li>
- </ul>
- <p>其它没有提到的 API 可以去 <a href="https://webpack.js.org/api/loaders/" target="_blank">Webpack 官网</a> 查看。 </p>
- <h2 id="加载本地-loader">加载本地 Loader</h2>
- <p>在开发 Loader 的过程中,为了测试编写的 Loader 是否能正常工作,需要把它配置到 Webpack 中后,才可能会调用该 Loader。
- 在前面的章节中,使用的 Loader 都是通过 Npm 安装的,要使用 Loader 时会直接使用 Loader 的名称,代码如下:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = {
- <span class="hljs-built_in">module</span>: {
- rules: [
- {
- test: <span class="hljs-regexp">/\.css$/</span>,
- use: [<span class="hljs-string">'style-loader'</span>],
- },
- ]
- },
- };
- </code></pre>
- <p>如果还采取以上的方法去使用本地开发的 Loader 将会很麻烦,因为你需要确保编写的 Loader 的源码是在 <code>node_modules</code> 目录下。
- 为此你需要先把编写的 Loader 发布到 Npm 仓库后再安装到本地项目使用。</p>
- <p>解决以上问题的便捷方法有两种,分别如下:</p>
- <h4 id="npm-link">Npm link</h4>
- <p>Npm link 专门用于开发和调试本地 Npm 模块,能做到在不发布模块的情况下,把本地的一个正在开发的模块的源码链接到项目的 <code>node_modules</code> 目录下,让项目可以直接使用本地的 Npm 模块。
- 由于是通过软链接的方式实现的,编辑了本地的 Npm 模块代码,在项目中也能使用到编辑后的代码。</p>
- <p>完成 Npm link 的步骤如下:</p>
- <ol>
- <li>确保正在开发的本地 Npm 模块(也就是正在开发的 Loader)的 <code>package.json</code> 已经正确配置好;</li>
- <li>在本地 Npm 模块根目录下执行 <code>npm link</code>,把本地模块注册到全局;</li>
- <li>在项目根目录下执行 <code>npm link loader-name</code>,把第2步注册到全局的本地 Npm 模块链接到项目的 <code>node_moduels</code> 下,其中的 <code>loader-name</code> 是指在第1步中的 <code>package.json</code> 文件中配置的模块名称。</li>
- </ol>
- <p>链接好 Loader 到项目后你就可以像使用一个真正的 Npm 模块一样使用本地的 Loader 了。</p>
- <h4 id="resolveloader">ResolveLoader</h4>
- <p>在 <a href="../2配置/2-7其它配置项.html#ResolveLoader">2-7其它配置项</a> 中曾介绍过 ResolveLoader 用于配置 Webpack 如何寻找 Loader。
- 默认情况下只会去 <code>node_modules</code> 目录下寻找,为了让 Webpack 加载放在本地项目中的 Loader 需要修改 <code>resolveLoader.modules</code>。</p>
- <p>假如本地的 Loader 在项目目录中的 <code>./loaders/loader-name</code> 中,则需要如下配置:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = {
- resolveLoader:{
- <span class="hljs-comment">// 去哪些目录下寻找 Loader,有先后顺序之分</span>
- modules: [<span class="hljs-string">'node_modules'</span>,<span class="hljs-string">'./loaders/'</span>],
- }
- }
- </code></pre>
- <p>加上以上配置后, Webpack 会先去 <code>node_modules</code> 项目下寻找 Loader,如果找不到,会再去 <code>./loaders/</code> 目录下寻找。</p>
- <h2 id="实战">实战</h2>
- <p>上面讲了许多理论,接下来从实际出发,来编写一个解决实际问题的 Loader。</p>
- <p>该 Loader 名叫 comment-require-loader,作用是把 JavaScript 代码中的注释语法</p>
- <pre><code class="lang-js"><span class="hljs-comment">// @require '../style/index.css'</span>
- </code></pre>
- <p>转换成</p>
- <pre><code class="lang-js"><span class="hljs-built_in">require</span>(<span class="hljs-string">'../style/index.css'</span>);
- </code></pre>
- <p>该 Loader 的使用场景是去正确加载针对 <a href="http://fis.baidu.com/fis3/docs/user-dev/require.html" target="_blank">Fis3</a> 编写的 JavaScript,这些 JavaScript 中存在通过注释的方式加载依赖的 CSS 文件。</p>
- <p>该 Loader 的使用方法如下:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = {
- <span class="hljs-built_in">module</span>: {
- rules: [
- {
- test: <span class="hljs-regexp">/\.js$/</span>,
- use: [<span class="hljs-string">'comment-require-loader'</span>],
- <span class="hljs-comment">// 针对采用了 fis3 CSS 导入语法的 JavaScript 文件通过 comment-require-loader 去转换 </span>
- include: [path.resolve(__dirname, <span class="hljs-string">'node_modules/imui'</span>)]
- }
- ]
- }
- };
- </code></pre>
- <p>该 Loader 的实现非常简单,完整代码如下:</p>
- <pre><code class="lang-js"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">replace</span>(<span class="hljs-params">source</span>) </span>{
- <span class="hljs-comment">// 使用正则把 // @require '../style/index.css' 转换成 require('../style/index.css'); </span>
- <span class="hljs-keyword">return</span> source.replace(<span class="hljs-regexp">/(\/\/ *@require) +(('|").+('|")).*/</span>, <span class="hljs-string">'require($2);'</span>);
- }
- <span class="hljs-built_in">module</span>.exports = <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">content</span>) </span>{
- <span class="hljs-keyword">return</span> replace(content);
- };
- </code></pre>
- <blockquote>
- <p>本实例<a href="https://github.com/gwuhaolin/comment-require-loader" target="_blank">提供项目完整代码</a></p>
- </blockquote>
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