为什么须要性能优化
在应用 Webpack 时，如果不留神性能优化，可能会产生性能问题，会导致在开发体验上不是十分丝滑，性能问题次要是编译速度慢，打包体积过大，因而性能优化也次要从这些方面来剖析。本文次要是本人平时的工作积攒和参考他人的文章，而进行总结，基于 Webpack4 版本。
构建剖析
编译速度剖析
对 Webpack 构建速度进行优化的首要任务就是去晓得哪些地方值得咱们留神。speed-measure-webpack-plugin 插件可能测量 Webpack 构建速度
SMP ⏱ General output time took 38.3 secs SMP ⏱ PluginsHtmlWebpackPlugin took 1.31 secsCopyPlugin took 0.016 secsOptimizeCssAssetsWebpackPlugin took 0.002 secsContextReplacementPlugin took 0.001 secsMiniCssExtractPlugin took 0 secsDefinePlugin took 0 secs SMP ⏱ Loaders_babel-loader@8.1.0@babel-loader took 29.98 secs module count = 1503_babel-loader@8.1.0@babel-loader, and _eslint-loader@3.0.4@eslint-loader took 18.74 secs module count = 86_css-loader@3.6.0@css-loader, and _less-loader@5.0.0@less-loader took 16.45 secs module count = 64modules with no loaders took 2.24 secs module count = 7_file-loader@5.1.0@file-loader took 1.03 secs module count = 17_style-loader@1.3.0@style-loader, and _css-loader@3.6.0@css-loader, and _less-loader@5.0.0@less-loader took 0.102 secs module count = 64_html-webpack-plugin@3.2.0@html-webpack-plugin took 0.021 secs module count = 1竟然达到了惊人的38.3秒，尽管有点不是很精确，然而十分慢。发现 babel-loader、eslint-loader、css-loader、less-loader 占据了大头。
const webpackBase = require('./webpack.base.conf');const path = require('path');const SpeedMeasureWebpackPlugin = require('speed-measure-webpack-plugin');const smp = new SpeedMeasureWebpackPlugin();module.exports = smp.wrap({ // 配置源码显示方式 devtool: 'eval-source-map', mode: 'development', entry: { app: ['./src/index.jsx'] }, output: { path: path.resolve(__dirname, 'dist'), filename: 'index.js' }, resolve: webpackBase.resolve, module: webpackBase.module, stats: webpackBase.stats, optimization: webpackBase.optimization, plugins: [ webpackBase.plugins.html, webpackBase.plugins.miniCssExtract, webpackBase.plugins.optimizeCssAssets, // webpackBase.plugins.progressBarPlugin, webpackBase.plugins.ContextReplacementPlugin, webpackBase.plugins.DefinePlugin, // webpackBase.plugins.AntdDayjsWebpackPlugin, webpackBase.plugins.CopyPlugin // webpackBase.plugins.HotModuleReplacementPlugin ], devServer: webpackBase.devServer, watchOptions: webpackBase.watchOptions, externals: webpackBase.externals});打包体积剖析
通过 webpack-bundle-analyzer 插件可能在 Webpack 构建完结后生成构建产物体积报告，配合可视化的页面，可能直观晓得产物中的具体占用体积。
const BundleAnalyzerPlugin = require('webpack-bundle-analyzer').BundleAnalyzerPlugin;module.exports = { plugins: bundleAnalyzer: new BundleAnalyzerPlugin({ analyzerPort: 8081 })],};效果图如下：
能够看出一个很显著的问题就是 ant、trtc、mobx 这些库，没有排除。
打包体积如下：
如何优化
放大构建指标
- 优化 resolve.modules 配置（缩小模块搜寻层级和不必要的编译工作）
- 优化 resolve.extensions 配置
- 减少缓存
const path = require('path');module.exports = { resolve: { // 主动解析确定的扩大 extensions: ['.js', '.jsx', '.css', '.less', '.json'], alias: { // 创立 import 或 require 的别名，来确保模块引入变得更简略 'react': path.resolve( __dirname ,'./node_modules/react/dist/react.min.js') }, // 当从 npm 包导入模块时，此选项将决定在 `package.json` 中应用哪个字段导入模块 // 默认值为 browser -> module -> main mainFields: ['main'] }, module: { rules: [ { // 排除node_modules模块 test: /\.(js|jsx)$/, exclude: /node_modules/, // 开启缓存 loader: 'babel-loader?cacheDirectory=true' } ] }};应用thread-loader，开启多过程
thread-loader 会将你的 loader 搁置在一个 worker 池外面运行，每个 worker 都是一个独自的有 600ms 限度的 node.js 过程。同时跨过程的数据交换也会被限度。请在高开销的 loader 中应用，否则成果不佳。
module.exports = { module: { rules: [ { test: /\.js$/, include: path.resolve('src'), use: [ 'thread-loader', // your expensive loader (e.g babel-loader) ], }, ], },};应用hard-source-webpack-plugin
在 Webpack4 中，hard-source-webpack-plugin 是 DLL 的更好替代者。
hard-source-webpack-plugin 是 Webpack 的插件，为模块提供两头缓存步骤。为了查看后果，您须要应用此插件运行 Webpack 两次：第一次构建将破费失常的工夫。第二次构建将显着放慢（大略晋升 90%的构建速度）。不过该插件很久没更新了，不太倡议应用。
去掉eslint-loader
因为我我的项目中应用了 eslint-loader 如果配置了 precommit，其实能够去掉的。
通过 externals 把相干的包，排除
Webpack
module.exports = { // externals 排除对应的包，注：排除掉的包必须要用script标签引入下 externals: { react: 'React', 'react-dom': 'ReactDOM', 'trtc-js-sdk': 'TRTC', bizcharts: 'BizCharts', antd: 'antd', mobx: 'mobx', 'mobx-react': 'mobxReact' }};index.html
<!DOCTYPE html><html lang="zh"> <head> <meta charset="utf-8" /> <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0" /> <meta name="baidu-site-verification" content="ptk9VJudKz" /> <link rel="stylesheet" href="https://xxx/antd.min3.26.20.css" /> <title>webpack</title> <script type="text/javascript" src="https://xxx/17.0.0react.production.min.js" ></script> <script type="text/javascript" src="https://xxx/17.0.0react-dom.production.min.js" ></script> <script type="text/javascript" src="https://xxx/BizCharts3.5.8.js" ></script> <script type="text/javascript" src="https://xxx/trtc4.6.7.js" ></script> <script type="text/javascript" src="https://xxx/moment2.29.1.min.js" ></script> <script type="text/javascript" src="https://xxx/moment2.29.1zh-cn.js" ></script> <script type="text/javascript" src="https://xxx/polyfill.min7.8.0.js" ></script> <script type="text/javascript" src="https://xxx/antd.min3.26.20.js" ></script> <script type="text/javascript" src="https://xxx/mobx.umd.min5.13.1.js" ></script> <script type="text/javascript" src="https://xxx/mobx-react.index.min5.4.4.js" ></script> </head> <body> <div id="root"></div> </body></html>JS压缩
从 Webpack4 开始，默认状况下应用 terser 压缩生产环境下的输入后果。Terser 是一款兼容 ES2015 + 的 JavaScript 压缩器。与 UglifyJS（许多我的项目的晚期规范）相比，它是面向未来的抉择。有一个 UglifyJS 的分支—— uglify-es，但因为它不再保护，于是就从这个分支诞生出了一个独立分支，它就是 terser。
const TerserPlugin = require('terser-webpack-plugin');module.exports = { optimization: { minimizer: [ // 压缩js new TerserPlugin({ test: /\.(jsx|js)$/, extractComments: true, parallel: true, cache: true }) ] },};CSS压缩
Webpack 4.0 当前，官网举荐应用 mini-css-extract-plugin 插件来打包 CSS 文件。
const MiniCssExtractPlugin = require('mini-css-extract-plugin');module.exports = { module: { rules: [ { test: /\.(css|less)$/, use: [MiniCssExtractPlugin.loader] } ] },};FAQ
ant 无奈加载
请确保加载程序，moment、polyfill 放在 ant 后面加载
mobx无奈加载
mobx引入 mobx.umd.min.js 库，mobx-react须要引入
package.json
{ "name": "webpack", "version": "1.0.0", "private": true, "main": "index.js", "dependencies": { "antd": "^3.26.20", "babel-eslint": "^10.0.3", "babel-loader": "^8.0.0", "babel-plugin-import": "^1.13.0", "babel-plugin-react-css-modules": "^5.2.6", "bizcharts": "^3.5.8", "china-division": "^2.3.1", "compression-webpack-plugin": "^3.0.1", "copy-webpack-plugin": "^5.1.1", "css-loader": "^3.2.0", "eslint": "^6.8.0", "eslint-config-prettier": "^6.11.0", "eslint-config-standard": "^14.1.0", "eslint-loader": "^3.0.4", "eslint-plugin-import": "^2.20.0", "eslint-plugin-promise": "^4.2.1", "eslint-plugin-react": "^7.17.0", "eslint-plugin-standard": "^4.0.1", "html-webpack-plugin": "^3.2.0", "less": "^3.8.1", "less-loader": "^5.0.0", "lint-staged": "^10.0.8", "mini-css-extract-plugin": "^0.8.0", "mobx": "^5.13.1", "mobx-react": "^5.4.4", "optimize-css-assets-webpack-plugin": "^5.0.1", "pre-commit": "^1.2.2", "progress-bar-webpack-plugin": "^1.12.1", "react": "^17.0.0", "react-dom": "^17.0.0", "speed-measure-webpack-plugin": "^1.3.1", "style-loader": "^1.2.1", "terser-webpack-plugin": "^2.2.1", "trtc-js-sdk": "^4.6.7", "viewerjs": "^1.5.0", "webpack": "^4.41.2", "webpack-bundle-analyzer": "^3.6.0", "webpack-cli": "^3.3.10", "webpack-dev-server": "^3.10.1" }}最终成果
打包体积：
打包体积由原先 2.1M 变成了 882KB，能够说成果十分微小。
包依赖：
ant、trtc、mobx 这些库也没了
编译速度：
SMP ⏱ General output time took 10.67 secs SMP ⏱ PluginsHtmlWebpackPlugin took 1.69 secsBundleAnalyzerPlugin took 0.091 secsCopyPlugin took 0.011 secsMiniCssExtractPlugin took 0.003 secsOptimizeCssAssetsWebpackPlugin took 0.002 secsDefinePlugin took 0.001 secsContextReplacementPlugin took 0 secs SMP ⏱ Loaders_babel-loader@8.1.0@babel-loader took 8.26 secs module count = 277_babel-loader@8.1.0@babel-loader, and _eslint-loader@3.0.4@eslint-loader took 7.18 secs module count = 86_css-loader@3.6.0@css-loader, and _less-loader@5.0.0@less-loader took 1.94 secs module count = 28modules with no loaders took 0.728 secs module count = 12_file-loader@5.1.0@file-loader took 0.392 secs module count = 17_style-loader@1.3.0@style-loader, and _css-loader@3.6.0@css-loader, and _less-loader@5.0.0@less-loader took 0.052 secs module count = 28_html-webpack-plugin@3.2.0@html-webpack-plugin took 0.026 secs module count = 1编译速度由原先 38.3 secs（理论编译速度大略15秒左右），缩小到 10.67 secs（理论编译速度10秒左右）。
国内外公共CDN地址
- BootCDN
- cdnjs
参考文献
- Webpack Guidebook
- Webpack 外围常识有哪些？
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