1. 多个 loader，但是每个文件只能匹配一个 loader，被一个 loader 处理，因此可以使用 oneOf 唯一匹配，不需要每个文件把所有的 loader 都询问一遍，可以提高 loader 的执行效率

2. 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‘)
 
process.env.NODE_ENV = ‘production‘
 
//复用loader
const commonCssLoader = [
    MiniCssExtractPlugin.loader,
    ‘css-loader‘,
    {
        loader:‘postcss-loader‘,
        options:{
            ident:‘postcss‘,
            plugins:()=>[require(‘postcss-preset-env‘)()]
        }
    }
]
 
module.exports={
    entry:‘./src/index.js‘,
    output:{
        filename:‘bundle.js‘,
        path:resolve(__dirname,‘build‘)
    },
    module:{
        rules:[
            {
                test: /\.js$/,
                exclude: /node_modules/,
                enforce: ‘pre‘, //优先执行，正常的，一个文件只能被一个loader处理，当一个文件要被多个loader处理，
                                //一定要指定loader执行的先后顺序，先执行eslint再执行babel
                loader: ‘eslint-loader‘,
                options: {
                    fix: true
                }
            },
            //以下loader中只会匹配一个，注意不能有两个loader处理同一种类型文件，所以eslint-loader放在oneOf匹配之前执行
            {
                oneOf:[
                    {
                        test: /\.css$/,
                        use: [...commonCssLoader]
                    },
                    {
                        test: /\.less$/,
                        use: [...commonCssLoader]
                    },
                    {
                        test: /\.js$/,
                        exclude: /node_modules/,
                        loader: ‘babel-loader‘,
                        options: {
                            presets: [
                                [‘@babel/preset-env‘, {
                                    useBuiltIns: ‘usage‘,
                                    corejs: { version: 3 },
                                    targets: {
                                        chrome: ‘60‘,
                                        firefox: ‘50‘
                                    }
                                }]
                            ]
                        }
                    },
                    {
                        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:‘bundle.css‘
        }),
        new OptimizeCssAssetsWebpackPlugin(),
        new HtmlWebpackPlugin({
            template:‘./src/index.html‘,
            minify:{
                collapseWhitespace:true,
                removeComments:true
            }
        })
    ],
    mode:‘production‘

webpack优化系列-oneOf