webpack打包优化之外部扩展(Externals)配置

基本使用

webpack可以使用externals配置,将某些模块不打包到输出文件中,从而使用用CDN引入外部模块

这样操作下来,不仅可以提高打包速度,还能优化网页加载性能。

index.html

<script  src="https://code.jquery.com/jquery-3.1.0.js"></script>

打开控制台,输入jQuery 可以看到是一个函数了

webpack打包优化之外部扩展(Externals)配置

webpack.config.js
module.exports = {
  externals: {
    jquery: 'jQuery',
  },
};

相当于告诉webpack, 当代码中引入jquery 的时候,使用window.jQuery 替换

import $ from 'jquery';
// const $ = jquery = window.jQuery

更多示例

module.exports = {
  externals: {
    moment: "moment",
    vue: "Vue",
    "element-ui": "ElementUI",
  },
};

完整示例

以Lodash 为例,体验一次webpack 的打包优化

初始化项目并安装依赖

$ node -v
v16.14.0
npm init -y
pnpm i -D cross-env webpack webpack-cli html-webpack-plugin
pnpm i -S lodash

项目目录结构

$ tree
.
├── package.json
├── src
│   └── index.js
└── webpack.config.js

package.json

{
  "name": "webpack-externals",
  "scripts": {
    "build": "cross-env NODE_ENV=production webpack --config webpack.config.js"
  },
  "devDependencies": {
    "cross-env": "^7.0.3",
    "html-webpack-plugin": "^5.5.0",
    "webpack": "^5.74.0",
    "webpack-cli": "^4.10.0"
  },
  "dependencies": {
    "lodash": "^4.17.21"
  }
}

webpack配置

// webpack.config.js
const HtmlWebpackPlugin = require("html-webpack-plugin");
module.exports = {
  entry: "./src/index.js",
  plugins: [new HtmlWebpackPlugin()],
};

示例代码

// src/index.js
import _ from 'lodash'
// 转换字符串string为驼峰写法。
let res = _.camelCase('get_username')
// 在控制台输出运行结果,证明lodash 库被正确引入
console.log(res);  // getUsername

打包编译

$ npm run build
> webpack-externals@1.0.0 build
> cross-env NODE_ENV=production webpack --config webpack.config.js
asset main.js 69.5 KiB [emitted] [minimized] (name: main) 1 related asset
asset index.html 214 bytes [compared for emit]
runtime modules 1010 bytes 5 modules
cacheable modules 531 KiB
  ./src/index.js 122 bytes [built] [code generated]
  ./node_modules/.pnpm/lodash@4.17.21/node_modules/lodash/lodash.js 531 KiB [built] [code generated]
webpack 5.74.0 compiled successfully in 2213 ms

使用externals 配置进行优化

$ tree
.
├── index.html     # 新增一个index文件
├── package.json
├── src
│   └── index.js
└── webpack.config.js

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Webpack Externals Demo</title>
  </head>
  <body>
    <!-- 从CDN引入外部依赖 -->
    <script src="https://cdn.bootcdn.net/ajax/libs/lodash.js/4.17.21/lodash.min.js"></script>
  </body>
</html>

修改配置文件

// webpack.config.js
const HtmlWebpackPlugin = require("html-webpack-plugin");
module.exports = {
  entry: "./src/index.js",
  // 配置外部依赖
  externals: {
    // lodash = window._
    lodash: "_",
  },
  plugins: [
    // 指定模板文件
    new HtmlWebpackPlugin({
      template: "./index.html",
    }),
  ],
};

webpack打包优化之外部扩展(Externals)配置

需要注意的是,两个地方出现的下划线 _ 并没有任何关系

// 此处的_ 表示的是局部变量赋值
// _ = lodash = window._
import _ from 'lodash'
// 表示的是指定引入库的时候,指定哪一个全局变量取替换
// lodash = window._
externals: {
    // lodash = window._
    lodash: "_",
},

打包编译

$ npm run build
> build
> cross-env NODE_ENV=production webpack --config webpack.config.js
asset index.html 380 bytes [emitted]
asset main.js 318 bytes [compared for emit] [minimized] (name: main)
runtime modules 663 bytes 3 modules
orphan modules 42 bytes [orphan] 1 module
./src/index.js + 1 modules 180 bytes [not cacheable] [built] [code generated]
webpack 5.74.0 compiled successfully in 475 ms

打包对比

webpack打包优化之外部扩展(Externals)配置

可以看到,使用了externals 外部引入之后,打包体积之和都比没有优化之前小,业务代码变得更小,能很好地利用js静态文件在浏览器中的缓存机制,更快的加载更新后的代码