tree shaking · 前端

树摇：去除没有使用的无用代码 webpack.config.js ``` const { resolve } = require('path'); const MiniCssExtractPlugin = require('mini-css-extract-plugin'); const OptimizeCssAssetsWebpackPlugin = require('optimize-css-assets-webpack-plugin'); const HtmlWebpackPlugin = require('html-webpack-plugin'); /* tree shaking：去除无用代码前提：1. 必须使用ES6模块化 2. 开启production环境作用: 减少代码体积在package.json中配置 "sideEffects": false 所有代码都没有副作用（都可以进行tree shaking）问题：可能会把css / @babel/polyfill （副作用）文件干掉 "sideEffects": ["*.css", "*.less"] */ // 定义nodejs环境变量：决定使用browserslist的哪个环境 process.env.NODE_ENV = 'production'; // 复用loader const commonCssLoader = [ MiniCssExtractPlugin.loader, 'css-loader', { // 还需要在package.json中定义browserslist loader: 'postcss-loader', options: { ident: 'postcss', plugins: () => [require('postcss-preset-env')()] } } ]; module.exports = { entry: './src/js/index.js', output: { filename: 'js/built.[contenthash:10].js', path: resolve(__dirname, 'build') }, module: { rules: [ { // 在package.json中eslintConfig --> airbnb test: /\.js$/, exclude: /node_modules/, // 优先执行 enforce: 'pre', loader: 'eslint-loader', options: { fix: true } }, { // 以下loader只会匹配一个 // 注意：不能有两个配置处理同一种类型文件 oneOf: [ { test: /\.css$/, use: [...commonCssLoader] }, { test: /\.less$/, use: [...commonCssLoader, 'less-loader'] }, /* 正常来讲，一个文件只能被一个loader处理。当一个文件要被多个loader处理，那么一定要指定loader执行的先后顺序：先执行eslint 在执行babel */ { test: /\.js$/, exclude: /node_modules/, loader: 'babel-loader', options: { presets: [ [ '@babel/preset-env', { useBuiltIns: 'usage', corejs: { version: 3 }, targets: { chrome: '60', firefox: '50' } } ] ], // 开启babel缓存 // 第二次构建时，会读取之前的缓存 cacheDirectory: true } }, { test: /\.(jpg|png|gif)/, loader: 'url-loader', options: { limit: 8 * 1024, name: '[hash:10].[ext]', outputPath: 'imgs', esModule: false } }, { test: /\.html$/, loader: 'html-loader' }, { exclude: /\.(js|css|less|html|jpg|png|gif)/, loader: 'file-loader', options: { outputPath: 'media' } } ] } ] }, plugins: [ new MiniCssExtractPlugin({ filename: 'css/built.[contenthash:10].css' }), new OptimizeCssAssetsWebpackPlugin(), new HtmlWebpackPlugin({ template: './src/index.html', minify: { collapseWhitespace: true, removeComments: true } }) ], mode: 'production', devtool: 'source-map' }; ``` src/index.html ``` <!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>webpack</title> </head> <body> <h1>hello cache</h1> </body> </html> ``` src/js/index.js ``` import { mul } from './test'; import '../css/index.css'; function sum(...args) { return args.reduce((p, c) => p + c, 0); } // eslint-disable-next-line console.log(mul(2, 3)); // eslint-disable-next-line console.log(sum(1, 2, 3, 4)); ``` src/js/test.js ``` export function mul(x, y) { return x * y; } export function count(x, y) { return x - y; } ``` src/css/index.css ``` html, body { margin: 0; padding: 0; height: 100%; background-color: deeppink; } ```