webpack高级概念，CSS文件的代码分割 与压缩(系列八)

我们之前写的css文件都会被打包进js文件中，要想把css单独打包成一个css文件该怎么做呢？

这个时候就需要用到 MiniCssExtractPlugin

开发环境用不到这个功能（因为这个东西不支持热模块更新，样式更改自动替换，不用刷新页面），一般都是用在生产环境中。

安装： npm install --save-dev mini-css-extract-plugin

如何将打包成的css文件压缩呢，需要用到optimize-css-assets-webpack-plugin

安装；npm install optimize-css-assets-webpack-plugin --save-dev 

webpack.dev.js，开发环境的style-loader不需要替换

const webpack = require('webpack');
const merge = require('webpack-merge');
const commonConfig = require('./webpack.common.js');

const devConfig = {
    mode: 'development',
    devtool: 'cheap-module-eval-source-map',
    devServer: {
        contentBase: './dist',
        open: true,
        port: 8080,
        hot: true
    },
    module: {
        rules: [{
            test: /.scss$/,
            use: [
                'style-loader', 
                {
                    loader: 'css-loader',
                    options: {
                        importLoaders: 2
                    }
                },
                'sass-loader',
                'postcss-loader'
            ]
        }, {
            test: /.css$/,
            use: [
                'style-loader',
                'css-loader',
                'postcss-loader'
            ]
        }]
    },
    plugins: [
        new webpack.HotModuleReplacementPlugin()
    ],
    // 树摇
    optimization: {
        usedExports: true
    }
}

module.exports = merge(commonConfig, devConfig);

webpack.prod.js, 生产环境，生产环境默认有树摇功能，对css模块导入会去除，需要在package.json中配置下，禁止树摇css模块

const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const OptimizeCSSAssetsPlugin = require("optimize-css-assets-webpack-plugin");
const merge = require('webpack-merge');
const commonConfig = require('./webpack.common.js');


const prodConfig = {
    mode: 'production',
    devtool: 'cheap-module-source-map',
    module: {
        rules:[{
            test: /.scss$/,
            use: [
                MiniCssExtractPlugin.loader,   //修改loader,将原来的style-lodader替换，不然没效果
                {
                    loader: 'css-loader',
                    options: {
                        importLoaders: 2
                    }
                },
                'sass-loader',
                'postcss-loader'
            ]
        }, {
            test: /.css$/,
            use: [

                MiniCssExtractPlugin.loader,
                'css-loader',
                'postcss-loader'
            ]
        }]
    },
    optimization: {
        minimizer: [new OptimizeCSSAssetsPlugin({})]
    },
    plugins: [
        new MiniCssExtractPlugin({   //配置插件，两个属性都是可选的,配置打包后css名称
            filename: '[name].css',
            chunkFilename: '[name].chunk.css'
        })
    ]
}

module.exports = merge(commonConfig, prodConfig);

package.json, 生产环境默认有树摇功能，对css模块导入会去除，需要在package.json中配置下，禁止树摇css模块

{
  "name": "lesson",
  "sideEffects": [
    "*.css"
  ],
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "dev-build": "webpack --config ./build/webpack.dev.js",
    "dev": "webpack-dev-server --config ./build/webpack.dev.js",
    "build": "webpack --config ./build/webpack.prod.js"
  },

index.js入口文件

import './style.css';
import './style1.css';

console.log('hello world');

style.css

body {
    background: green;
}

style1.css

body {
    font-size:  100px;
}

这样打包（npm  run  build）之后，css会被单独打包成一个css文件。并且代码是压缩的