前言

CommonsChunkPlugin主要是用来提取第三方库和公共模块，避免首屏加载的bundle文件或者按需加载的bundle文件体积过大，从而导致加载时间过长，着实是优化的一把利器。

先来说一下各种教程以及文档中CommonsChunkPlugin提及到chunk有哪几种，主要有以下三种：

1. webpack当中配置的入口文件（entry）是chunk，可以理解为entry chunk
2. 入口文件以及它的依赖文件通过code split（代码分割）出来的也是chunk，可以理解为children chunk
3. 通过CommonsChunkPlugin创建出来的文件也是chunk，可以理解为commons chunk

splitChunks优点

• 它不会打包不需要的模块
• 对异步模块有效（默认情况下是打开的）
• 更加容易使用和更加自动化

实践

src目录下文件，如下：

common.js
export const common = '公共文件';

first.js
import {common} from './common';
import $ from 'jquery';
console.log($，`第一个文件，引入公共文件：${common}`);

second.js
import {common} from './common';
import $ from 'jquery';
console.log($，`second ${common}`

webpack.config.js：

const path = require("path");
const webpack = require("webpack");

const config = {
    entry: {
        first: './src/first.js',
        second: './src/second.js'
    },
    output: {
        path: path.resolve(__dirname,'./dist'),
        filename: '[name].js',
        clean: true
    },
}

module.exports = config;

运行 webpack

asset first.js 324 KiB [emitted] (name: first)
asset second.js 324 KiB [emitted] [compared for emit] (name: second)
runtime modules 1.95 KiB 8 modules
cacheable modules 282 KiB
  ./src/first.js 125 bytes [built] [code generated]
  ./src/second.js 91 bytes [built] [code generated]
  ./src/common.js 37 bytes [built] [code generated]
  ./node_modules/jquery/dist/jquery.js 282 KiB [built] [code generated]
webpack 5.73.0 compiled successfully in 371 ms

dist文件下 生成两个文件first.js, second.js first.js看下：

可以看到first.js和second.js里面把jquery, common.js , webpack.runtime文件全部引入进来了,这样肯定重复了，需要把公共模块抽出来

分离出第三方库，自定义的公共模块，webpack运行文件

const path = require("path");
const webpack = require("webpack");

const config = {
    entry: {
        first: './src/first.js',
        second: './src/second.js'
    },
    output: {
        path: path.resolve(__dirname,'./dist'),
        filename: '[name].js',
        clean: true
    },
    optimization:{
      splitChunks: {
       // 选择哪些 chunk 进行优化，可选值：initial(初始块)、async(按需加载块)、all(全部块)，默认为all,
        chunks: 'all',
      },
      
    }
}
module.exports = config;

查看运行情况：

asset vendors-node_modules_jquery_dist_jquery_js.js 320 KiB [emitted] (id hint: vendors)
asset first.js 8.94 KiB [emitted] (name: first)
asset second.js 8.92 KiB [emitted] (name: second)
Entrypoint first 329 KiB = vendors-node_modules_jquery_dist_jquery_js.js 320 KiB first.js 8.94 KiB
Entrypoint second 329 KiB = vendors-node_modules_jquery_dist_jquery_js.js 320 KiB second.js 8.92 KiB
runtime modules 7 KiB 12 modules
cacheable modules 282 KiB
  ./src/first.js 125 bytes [built] [code generated]
  ./src/second.js 91 bytes [built] [code generated]
  ./src/common.js 37 bytes [built] [code generated]
  ./node_modules/jquery/dist/jquery.js 282 KiB [built] [code generated]
webpack 5.73.0 compiled successfully in 371 ms

查看dist目录下，新增了一个vendor*.js的文件，把jquery 单独提取出来了

最终代码：

const path = require("path");
const webpack = require("webpack");

const config = {
    entry: {
        first: './src/first.js',
        second: './src/second.js'
    },
    output: {
        path: path.resolve(__dirname,'./dist'),
        filename: '[name].js',
        clean: true
    },
    optimization:{
      runtimeChunk: {
        name: 'runtime'
      },
      splitChunks: {
        cacheGroups: {
          commons: {
            name: 'commons', // 设置模块名称，如果不设置就是commons_node_modules_jquery_jquery_src_common_js.js
            chunks: 'all',
            minChunks: 2,
          },
        },
      },
    }
}
module.exports = config;

运行结果：

asset commons.js 322 KiB [emitted] (name: commons) (id hint: commons)
asset runtime.js 7.15 KiB [compared for emit] (name: runtime)
asset second.js 1.02 KiB [emitted] (name: second)
asset first.js 1.01 KiB [emitted] (name: first)
Entrypoint first 330 KiB = runtime.js 7.15 KiB commons.js 322 KiB first.js 1.01 KiB
Entrypoint second 330 KiB = runtime.js 7.15 KiB commons.js 322 KiB second.js 1.02 KiB
runtime modules 3.5 KiB 6 modules
cacheable modules 282 KiB
  ./src/first.js 125 bytes [built] [code generated]
  ./src/second.js 91 bytes [built] [code generated]
  ./src/common.js 37 bytes [built] [code generated]
  ./node_modules/jquery/dist/jquery.js 282 KiB [built] [code generated]
webpack 5.73.0 compiled successfully in 350 ms

生成文件：