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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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- 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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- <li class="chapter " data-level="1.6.1" data-path="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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- 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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- 4-9 CDN 加速
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- <h1 id="4-11-提取公共代码">4-11 提取公共代码</h1>
- <h2 id="为什么需要提取公共代码">为什么需要提取公共代码</h2>
- <p>大型网站通常会由多个页面组成,每个页面都是一个独立的单页应用。
- 但由于所有页面都采用同样的技术栈,以及使用同一套样式代码,这导致这些页面之间有很多相同的代码。</p>
- <p>如果每个页面的代码都把这些公共的部分包含进去,会造成以下问题:</p>
- <ul>
- <li>相同的资源被重复的加载,浪费用户的流量和服务器的成本;</li>
- <li>每个页面需要加载的资源太大,导致网页首屏加载缓慢,影响用户体验。</li>
- </ul>
- <p>如果把多个页面公共的代码抽离成单独的文件,就能优化以上问题。
- 原因是假如用户访问了网站的其中一个网页,那么访问这个网站下的其它网页的概率将非常大。
- 在用户第一次访问后,这些页面公共代码的文件已经被浏览器缓存起来,在用户切换到其它页面时,存放公共代码的文件就不会再重新加载,而是直接从缓存中获取。
- 这样做后有如下好处:</p>
- <ul>
- <li>减少网络传输流量,降低服务器成本;</li>
- <li>虽然用户第一次打开网站的速度得不到优化,但之后访问其它页面的速度将大大提升。</li>
- </ul>
- <h2 id="如何提取公共代码">如何提取公共代码</h2>
- <p>你已经知道了提取公共代码会有什么好处,但是在实战中具体要怎么做,以达到效果最优呢?
- 通常你可以采用以下原则去为你的网站提取公共代码:</p>
- <ul>
- <li>根据你网站所使用的技术栈,找出网站所有页面都需要用到的基础库,以采用 React 技术栈的网站为例,所有页面都会依赖 react、react-dom 等库,把它们提取到一个单独的文件。
- 一般把这个文件叫做 <code>base.js</code>,因为它包含所有网页的基础运行环境;</li>
- <li>在剔除了各个页面中被 <code>base.js</code> 包含的部分代码外,再找出所有页面都依赖的公共部分的代码提取出来放到 <code>common.js</code> 中去。</li>
- <li>再为每个网页都生成一个单独的文件,这个文件中不再包含 <code>base.js</code> 和 <code>common.js</code> 中包含的部分,而只包含各个页面单独需要的部分代码。</li>
- </ul>
- <p>文件之间的结构图如下:</p>
- <p><img src="img/4-11提取公共代码.png" alt="图4-11 提取公共代码文件结构图"></p>
- <p>读到这里你可以会有疑问:既然能找出所有页面都依赖的公共代码,并提取出来放到 <code>common.js</code> 中去,为什么还需要再把网站所有页面都需要用到的基础库提取到 <code>base.js</code> 去呢?
- 原因是为了长期的缓存 <code>base.js</code> 这个文件。</p>
- <p>发布到线上的文件都会采用在<a href="4-9CDN加速.html">4-9CDN加速</a>中介绍过的方法,对静态文件的文件名都附加根据文件内容计算出 Hash 值,也就是最终 <code>base.js</code> 的文件名会变成 <code>base_3b1682ac.js</code>,以长期缓存文件。
- 网站通常会不断的更新发布,每次发布都会导致 <code>common.js</code> 和各个网页的 JavaScript 文件都会因为文件内容发生变化而导致其 Hash 值被更新,也就是缓存被更新。</p>
- <p>把所有页面都需要用到的基础库提取到 <code>base.js</code> 的好处在于只要不升级基础库的版本,<code>base.js</code> 的文件内容就不会变化,Hash 值不会被更新,缓存就不会被更新。
- 每次发布浏览器都会使用被缓存的 <code>base.js</code> 文件,而不用去重新下载 <code>base.js</code> 文件。
- 由于 <code>base.js</code> 通常会很大,这对提升网页加速速度能起到很大的效果。</p>
- <h2 id="如何通过-webpack-提取公共代码">如何通过 Webpack 提取公共代码</h2>
- <p>你已经知道如何提取公共代码,接下来教你如何用 Webpack 实现。</p>
- <p>Webpack 内置了专门用于提取多个 Chunk 中公共部分的插件 CommonsChunkPlugin,CommonsChunkPlugin 大致使用方法如下:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">const</span> CommonsChunkPlugin = <span class="hljs-built_in">require</span>(<span class="hljs-string">'webpack/lib/optimize/CommonsChunkPlugin'</span>);
- <span class="hljs-keyword">new</span> CommonsChunkPlugin({
- <span class="hljs-comment">// 从哪些 Chunk 中提取</span>
- chunks: [<span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>],
- <span class="hljs-comment">// 提取出的公共部分形成一个新的 Chunk,这个新 Chunk 的名称</span>
- name: <span class="hljs-string">'common'</span>
- })
- </code></pre>
- <p>以上配置就能从网页 A 和网页 B 中抽离出公共部分,放到 common 中。</p>
- <p>每个 CommonsChunkPlugin 实例都会生成一个新的 Chunk,这个新 Chunk 中包含了被提取出的代码,在使用过程中必须指定 <code>name</code> 属性,以告诉插件新生成的 Chunk 的名称。
- 其中 <code>chunks</code> 属性指明从哪些已有的 Chunk 中提取,如果不填该属性,则默认会从所有已知的 Chunk 中提取。</p>
- <blockquote>
- <p>Chunk 是一系列文件的集合,一个 Chunk 中会包含这个 Chunk 的入口文件和入口文件依赖的文件。</p>
- </blockquote>
- <p>通过以上配置输出的 common Chunk 中会包含所有页面都依赖的基础运行库 react、react-dom,为了把基础运行库从 common 中抽离到 base 中去,还需要做一些处理。</p>
- <p>首先需要先配置一个 Chunk,这个 Chunk 中只依赖所有页面都依赖的基础库以及所有页面都使用的样式,为此需要在项目中写一个文件 <code>base.js</code> 来描述 base Chunk 所依赖的模块,文件内容如下:</p>
- <pre><code class="lang-js"><span class="hljs-comment">// 所有页面都依赖的基础库</span>
- <span class="hljs-keyword">import</span> <span class="hljs-string">'react'</span>;
- <span class="hljs-keyword">import</span> <span class="hljs-string">'react-dom'</span>;
- <span class="hljs-comment">// 所有页面都使用的样式</span>
- <span class="hljs-keyword">import</span> <span class="hljs-string">'./base.css'</span>;
- </code></pre>
- <p>接着再修改 Webpack 配置,在 entry 中加入 base,相关修改如下:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = {
- entry: {
- base: <span class="hljs-string">'./base.js'</span>
- },
- };
- </code></pre>
- <p>以上就完成了对新 Chunk base 的配置。</p>
- <p>为了从 common 中提取出 base 也包含的部分,还需要配置一个 CommonsChunkPlugin,相关代码如下:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">new</span> CommonsChunkPlugin({
- <span class="hljs-comment">// 从 common 和 base 两个现成的 Chunk 中提取公共的部分</span>
- chunks: [<span class="hljs-string">'common'</span>, <span class="hljs-string">'base'</span>],
- <span class="hljs-comment">// 把公共的部分放到 base 中</span>
- name: <span class="hljs-string">'base'</span>
- })
- </code></pre>
- <p>由于 common 和 base 公共的部分就是 base 目前已经包含的部分,所以这样配置后 common 将会变小,而 base 将保持不变。</p>
- <p>以上都配置好后重新执行构建,你将会得到四个文件,它们分别是:</p>
- <ul>
- <li><code>base.js</code>:所有网页都依赖的基础库组成的代码;</li>
- <li><code>common.js</code>:网页A、B都需要的,但又不在 <code>base.js</code> 文件中出现过的代码;</li>
- <li><code>a.js</code>:网页 A 单独需要的代码;</li>
- <li><code>b.js</code>:网页 B 单独需要的代码。</li>
- </ul>
- <p>为了让网页正常运行,以网页 A 为例,你需要在其 HTML 中按照以下顺序引入以下文件才能让网页正常运行:</p>
- <pre><code class="lang-html"><span class="hljs-tag"><<span class="hljs-name">script</span> <span class="hljs-attr">src</span>=<span class="hljs-string">"base.js"</span>></span><span class="undefined"></span><span class="hljs-tag"></<span class="hljs-name">script</span>></span>
- <span class="hljs-tag"><<span class="hljs-name">script</span> <span class="hljs-attr">src</span>=<span class="hljs-string">"common.js"</span>></span><span class="undefined"></span><span class="hljs-tag"></<span class="hljs-name">script</span>></span>
- <span class="hljs-tag"><<span class="hljs-name">script</span> <span class="hljs-attr">src</span>=<span class="hljs-string">"a.js"</span>></span><span class="undefined"></span><span class="hljs-tag"></<span class="hljs-name">script</span>></span>
- </code></pre>
- <p>以上就完成了提取公共代码需要的所有步骤。</p>
- <p>针对 CSS 资源,以上理论和方法同样有效,也就是说你也可以对 CSS 文件做同样的优化。</p>
- <p>以上方法可能会出现 <code>common.js</code> 中没有代码的情况,原因是去掉基础运行库外很难再找到所有页面都会用上的模块。
- 在出现这种情况时,你可以采取以下做法之一:</p>
- <ul>
- <li>CommonsChunkPlugin 提供一个选项 <code>minChunks</code>,表示文件要被提取出来时需要在指定的 Chunks 中最小出现最小次数。
- 假如 <code>minChunks=2</code>、<code>chunks=['a','b','c','d']</code>,任何一个文件只要在 <code>['a','b','c','d']</code> 中任意两个以上的 Chunk 中都出现过,这个文件就会被提取出来。
- 你可以根据自己的需求去调整 minChunks 的值,minChunks 越小越多的文件会被提取到 <code>common.js</code> 中去,但这也会导致部分页面加载的不相关的资源越多;
- minChunks 越大越少的文件会被提取到 <code>common.js</code> 中去,但这会导致 <code>common.js</code> 变小、效果变弱。</li>
- <li>根据各个页面之间的相关性选取其中的部分页面用 CommonsChunkPlugin 去提取这部分被选出的页面的公共部分,而不是提取所有页面的公共部分,而且这样的操作可以叠加多次。
- 这样做的效果会很好,但缺点是配置复杂,你需要根据页面之间的关系去思考如何配置,该方法不通用。</li>
- </ul>
- <blockquote>
- <p>本实例<a href="http://webpack.wuhaolin.cn/4-11提取公共代码.zip" target="_blank">提供项目完整代码</a></p>
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- <script src="../gitbook/gitbook-plugin-adsense/plugin.js"></script>
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- <script src="../gitbook/gitbook-plugin-lunr/lunr.min.js"></script>
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- <script src="../gitbook/gitbook-plugin-lunr/search-lunr.js"></script>
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- <script src="../gitbook/gitbook-plugin-sharing/buttons.js"></script>
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- <script src="../gitbook/gitbook-plugin-fontsettings/fontsettings.js"></script>
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- </body>
- </html>
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