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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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- 3-5 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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- 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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- 第4章 优化
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- 4-1 缩小文件搜索范围
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- 4-2 使用 DllPlugin
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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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- 4-6 开启模块热替换
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- 4-7 区分环境
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- 4-8 压缩代码
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- <li class="chapter " data-level="1.6.9" data-path="../4优化/4-9CDN加速.html">
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- 4-9 CDN 加速
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- <li class="chapter " data-level="1.6.10" data-path="../4优化/4-10使用TreeShaking.html">
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- 4-10 使用 Tree Shaking
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- <h1 id="3-1-使用-es6-语言">3-1 使用 ES6 语言</h1>
- <p>ECMAScript 6.0 是2015年发布的下一代 JavaScript 语言标准,它引入了新的语法和 API 来提升开发效率。</p>
- <p></p><p>想学习更多 ES6 的新特性,推荐阅读阮一峰的《ECMAScript 6 入门》:</p>
- <a href="http://union-click.jd.com/jdc?d=Qh76Pi" target="_blank">
- <img src="http://es6.ruanyifeng.com/images/cover_thumbnail_3rd.jpg">
- </a><p></p>
- <p>虽然目前部分浏览器和 Node.js 已经支持 ES6,但由于它们对 ES6 所有的标准支持不全,这导致在开发中不敢全面地使用 ES6。</p>
- <p>通常我们需要把采用 ES6 编写的代码转换成目前已经支持良好的 ES5 代码,这包含2件事:</p>
- <ol>
- <li>把新的 ES6 语法用 ES5 实现,例如 ES6 的 <code>class</code> 语法用 ES5 的 <code>prototype</code> 实现。</li>
- <li>给新的 API 注入 polyfill ,例如项目使用 <code>fetch</code> API 时,只有注入对应的 polyfill 后,才能在低版本浏览器中正常运行。</li>
- </ol>
- <h2 id="babel">Babel</h2>
- <p><a href="https://babeljs.io" target="_blank">Babel</a> 可以方便的完成以上2件事。
- Babel 是一个 JavaScript 编译器,能将 ES6 代码转为 ES5 代码,让你使用最新的语言特性而不用担心兼容性问题,并且可以通过插件机制根据需求灵活的扩展。
- 在 Babel 执行编译的过程中,会从项目根目录下的 <code>.babelrc</code> 文件读取配置。<code>.babelrc</code> 是一个 JSON 格式的文件,内容大致如下:</p>
- <pre><code class="lang-json">{
- <span class="hljs-string">"plugins"</span>: [
- [
- <span class="hljs-string">"transform-runtime"</span>,
- {
- <span class="hljs-string">"polyfill"</span>: <span class="hljs-literal">false</span>
- }
- ]
- ],
- <span class="hljs-string">"presets"</span>: [
- [
- <span class="hljs-string">"es2015"</span>,
- {
- <span class="hljs-string">"modules"</span>: <span class="hljs-literal">false</span>
- }
- ],
- <span class="hljs-string">"stage-2"</span>,
- <span class="hljs-string">"react"</span>
- ]
- }
- </code></pre>
- <h3 id="plugins">Plugins</h3>
- <p><code>plugins</code> 属性告诉 Babel 要使用哪些插件,插件可以控制如何转换代码。</p>
- <p>以上配置文件里的 <code>transform-runtime</code> 对应的插件全名叫做 <code>babel-plugin-transform-runtime</code>,即在前面加上了 <code>babel-plugin-</code>,要让 Babel 正常运行我们必须先安装它:</p>
- <pre><code class="lang-bash">npm i -D babel-plugin-transform-runtime
- </code></pre>
- <p><code>babel-plugin-transform-runtime</code> 是 Babel 官方提供的一个插件,作用是减少冗余代码。
- Babel 在把 ES6 代码转换成 ES5 代码时通常需要一些 ES5 写的辅助函数来完成新语法的实现,例如在转换 <code>class extent</code> 语法时会在转换后的 ES5 代码里注入 <code>_extent</code> 辅助函数用于实现继承:</p>
- <pre><code class="lang-js"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">_extent</span>(<span class="hljs-params">target</span>) </span>{
- <span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i = <span class="hljs-number">1</span>; i < <span class="hljs-built_in">arguments</span>.length; i++) {
- <span class="hljs-keyword">var</span> source = <span class="hljs-built_in">arguments</span>[i];
- <span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> key <span class="hljs-keyword">in</span> source) {
- <span class="hljs-keyword">if</span> (<span class="hljs-built_in">Object</span>.prototype.hasOwnProperty.call(source, key)) {
- target[key] = source[key];
- }
- }
- }
- <span class="hljs-keyword">return</span> target;
- }
- </code></pre>
- <p>这会导致每个使用了 <code>class extent</code> 语法的文件都被注入重复的<code>_extent</code> 辅助函数代码,<code>babel-plugin-transform-runtime</code> 的作用在于不把辅助函数内容注入到文件里,而是注入一条导入语句:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">var</span> _extent = <span class="hljs-built_in">require</span>(<span class="hljs-string">'babel-runtime/helpers/_extent'</span>);
- </code></pre>
- <p>这样能减小 Babel 编译出来的代码的文件大小。</p>
- <p>同时需要注意的是由于 <code>babel-plugin-transform-runtime</code> 注入了 <code>require('babel-runtime/helpers/_extent')</code> 语句到编译后的代码里,需要安装 <code>babel-runtime</code> 依赖到你的项目后,代码才能正常运行。
- 也就是说 <code>babel-plugin-transform-runtime</code> 和 <code>babel-runtime</code> 需要配套使用,使用了 <code>babel-plugin-transform-runtime</code> 后一定需要 <code>babel-runtime</code>。</p>
- <h3 id="presets">Presets</h3>
- <p><code>presets</code> 属性告诉 Babel 要转换的源码使用了哪些新的语法特性,一个 Presets 对一组新语法特性提供支持,多个 Presets 可以叠加。
- Presets 其实是一组 Plugins 的集合,每一个 Plugin 完成一个新语法的转换工作。Presets 是按照 ECMAScript 草案来组织的,通常可以分为以下三大类:</p>
- <ol>
- <li><p>已经被写入 ECMAScript 标准里的特性,由于之前每年都有新特性被加入到标准里,所以又可细分为:</p>
- <ul>
- <li><a href="https://babeljs.io/docs/plugins/preset-es2015/" target="_blank">es2015</a> 包含在2015里加入的新特性;</li>
- <li><a href="https://babeljs.io/docs/plugins/preset-es2016/" target="_blank">es2016</a> 包含在2016里加入的新特性;</li>
- <li><a href="https://babeljs.io/docs/plugins/preset-es2017/" target="_blank">es2017</a> 包含在2017里加入的新特性;</li>
- <li><a href="https://babeljs.io/docs/plugins/preset-env/" target="_blank">env</a> 包含当前所有 ECMAScript 标准里的最新特性。</li>
- </ul>
- <p>它们之间的关系如图:</p>
- <p><img src="img/3-1presets-es.png" alt="图3.1.1 ECMAScript 标准里的特性关系图"></p>
- </li>
- <li><p>被社区提出来的但还未被写入 ECMAScript 标准里特性,这其中又分为以下四种:</p>
- <ul>
- <li><a href="https://babeljs.io/docs/plugins/preset-stage-0/" target="_blank">stage0</a> 只是一个美好激进的想法,有 Babel 插件实现了对这些特性的支持,但是不确定是否会被定为标准;</li>
- <li><a href="https://babeljs.io/docs/plugins/preset-stage-1/" target="_blank">stage1</a> 值得被纳入标准的特性;</li>
- <li><a href="https://babeljs.io/docs/plugins/preset-stage-2/" target="_blank">stage2</a> 该特性规范已经被起草,将会被纳入标准里;</li>
- <li><a href="https://babeljs.io/docs/plugins/preset-stage-3/" target="_blank">stage3</a> 该特性规范已经定稿,各大浏览器厂商和 Node.js 社区开始着手实现;</li>
- <li>stage4 在接下来的一年将会加入到标准里去。</li>
- </ul>
- <p>它们之间的关系如图:</p>
- <p><img src="img/3-1presets-stage.png" alt="图3.1.2 stage 关系图"></p>
- </li>
- </ol>
- <ol>
- <li>为了支持一些特定应用场景下的语法,和 ECMAScript 标准没有关系,例如 <code>babel-preset-react</code> 是为了支持 React 开发中的 JSX 语法。</li>
- </ol>
- <p>在实际应用中,你需要根据项目源码所使用的语法去安装对应的 Plugins 或 Presets。</p>
- <h2 id="接入-babel">接入 Babel</h2>
- <p>在了解 Babel 后,下一步要知道如何在 Webpack 中使用它。
- 由于 Babel 所做的事情是转换代码,所以应该通过 Loader 去接入 Babel,Webpack 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: [
- {
- test: <span class="hljs-regexp">/\.js$/</span>,
- use: [<span class="hljs-string">'babel-loader'</span>],
- },
- ]
- },
- <span class="hljs-comment">// 输出 source-map 方便直接调试 ES6 源码</span>
- devtool: <span class="hljs-string">'source-map'</span>
- };
- </code></pre>
- <p>配置命中了项目目录下所有的 JavaScript 文件,通过 <code>babel-loader</code> 去调用 Babel 完成转换工作。
- 在重新执行构建前,需要先安装新引入的依赖:</p>
- <pre><code class="lang-bash"><span class="hljs-comment"># Webpack 接入 Babel 必须依赖的模块</span>
- npm i -D babel-core babel-loader
- <span class="hljs-comment"># 根据你的需求选择不同的 Plugins 或 Presets</span>
- npm i -D babel-preset-env
- </code></pre>
- <blockquote>
- <p>本实例<a href="http://webpack.wuhaolin.cn/3-1使用ES6语言.zip" target="_blank">提供项目完整代码</a></p>
- </blockquote>
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