loader的概念。那这篇文章继续介绍一些常用loader，并展现它的强大之处处理less
less与sass的功能都一样，这里我就以less为例，介绍其对应loader的用法。less的语法对于浏览器那是必需不认识的，要处理成浏览器认识的需要借助两个模块，一个less和一个less-loader
安装
npm i less less-loader -D
创建文件
结构目录如下图：index.css内容如下：
#box{ ???width: 800px; ???height: 500px; ???border: 5px solid #999; ???color: #00f; ???/*background: green;*/ ???background: url(../images/img_01.jpg);}index.js内容如下：
import ‘../css/index.css‘; ?//引入css文件import ‘../less/less.less‘; //引入less文件import img from ‘../images/img_01.jpg‘;const newImg=new Image();newImg.onload=()=>document.body.appendChild(newImg);newImg.src=img;less.less内容如下
//如果对less不懂的同学，可以加我微信：kaivon@w:200px;@border:1px solid #f00;#box{ ???width: @w; ???border: @border; ???ul{ ???????margin: 0; ???????padding: 0; ???????list-style: none; ???????li{ ???????????height: 30px; ???????????background: #ccc; ???????????a{ ?????????????color: #f00; ?????????????&:hover{ ?//&为上一层选择器 ?????????????????color: blue; ?????????????} ???????????} ???????} ???} }template.html内容如下：
<!DOCTYPE html><html> ???<head> ???????<meta charset="UTF-8"> ???????<title><%= htmlWebpackPlugin.options.title %></title> ???</head> ???<body> ???????<div id="box"> ???????????<p>我是陈学辉</p> ???????????<ul> ???????????????<li><a href="#">red</a></li> ???????????????<li><a href="#">green</a></li> ???????????????<li><a href="#">blue</a></li> ???????????</ul> ???????</div> ???</body></html>package.json内容如下：
{ ?"name": "webpack-demo", ?"version": "1.0.0", ?"description": "", ?"main": "webpack.config.js", ?"scripts": { ???"build": "webpack --mode production", ???"dev": "webpack-dev-server --mode development" ?}, ?"keywords": [], ?"author": "", ?"license": "ISC", ?"devDependencies": { ???"css-loader": "^1.0.0", ???"file-loader": "^1.1.11", ???"html-webpack-plugin": "^3.2.0", ???"less": "^3.8.1", ???"less-loader": "^4.1.0", ???"mini-css-extract-plugin": "^0.4.1", ???"url-loader": "^1.0.1", ???"webpack": "^4.16.3", ???"webpack-cli": "^3.1.0", ???"webpack-dev-server": "^3.1.5" ?}}webpack.config.js内容如下：
const path=require(‘path‘);const HtmlWebpackPlugin=require(‘html-webpack-plugin‘);const MiniCssExtractPlugin=require(‘mini-css-extract-plugin‘);module.exports={ ???entry:‘./src/js/index.js‘, ???output:{ ???????path:path.resolve(__dirname,‘dist‘), ???????filename:‘js/index.js‘ ???}, ???plugins:[ ???????new HtmlWebpackPlugin({ ???????????title:‘陈学辉‘, ???????????template:‘./src/template.html‘, ???????????filename:‘index.html‘ ???????}), ???????new MiniCssExtractPlugin({ ???????????filename:‘css/index.css‘ ???????}), ???], ???devServer:{ ???????host:‘localhost‘, ???????port:1573, ???????open:true ???}, ???module:{ ???????rules:[ ???????????{ ???????????????test:/\.css$/, ???????????????use:[ ???????????????????{ ???????????????????????loader:MiniCssExtractPlugin.loader, ???????????????????????options:{ ???????????????????????????publicPath:‘../‘ ???????????????????????} ???????????????????}, ???????????????????‘css-loader‘, ???????????????] ???????????}, ???????????{ ???????????????test:/\.less$/, ???????????????use:[ ??//把less编译到css文件里 ???????????????????{ ???????????????????????loader:MiniCssExtractPlugin.loader, ???????????????????????options:{ ???????????????????????????publicPath:‘../‘ ???????????????????????} ???????????????????}, ???????????????????‘css-loader‘, ??//注意顺序 ???????????????????‘less-loader‘ ???????????????] ???????????}, ???????????{ ???????????????test:/\.(jpg|png|gif)$/, ???????????????use:[ ???????????????????{ ???????????????????????loader:‘url-loader‘, ???????????????????????options:{ ???????????????????????????limit:5 * 1024, ???????????????????????????outputPath:‘images‘ ???????????????????????} ???????????????????} ???????????????] ???????????} ???????] ???}}执行命令npm run dev后浏览器里的内容正常显示，less文件里的代码被编译成真正的css代码，同时利用MiniCssExtractPlugin.loader与css一起合并到了index.css文件里。
处理ES6与React
ES6的语法大部分已经被各在浏览器所支持，当然除了万恶的IE，但是部分新增API很遗憾并不被浏览器所支持：比如内置对象新增的一些方法、Set和Map对象、Generator等。React与Vue都支持一种语法叫JSX，这种语法也是不被浏览器所支持的，对于这两个的处理我选择用Babel，Babel就是为了处理这哥俩而生的。关于Babel的用法可以参考我录制的视频课程“NPM与模块化开发”
ES6
npm i babel-loader babel-core babel-preset-env babel-polyfill -D
- babel-loader : 处理ES6或者React的loader
- babel-core : babel运行的核心内容
- babel-preset-env : 根据代码自动选择版本（ES的版本有很多，env表示会自动选择版本）
- babel-polyfill : 用于处理新增的API
React
npm i react react-dom babel-preset-react -D
- react : react的核心内容
- react-dom : 处理react里的dom
- babel-preset-react : babel处理react的组件
index.js内容如下
import ‘../css/index.css‘;import ‘../less/less.less‘;import img from ‘../images/img_01.jpg‘;import ‘babel-polyfill‘; ???//处理ES6新增的API，需要导入这个模块import React from ‘react‘; ?//reactimport ReactDOM from ‘react-dom‘; ??//reactconst newImg=new Image();newImg.onload=()=>{ ???document.body.appendChild(newImg);};newImg.src=img;//ES6const fn=()=>console.log(123);//map对象const map=new Map();map.set(‘name‘,‘kaivon‘);console.log(map);//Array.from方法const strArr=Array.from(‘kaivon‘);console.log(strArr);//Object.assign方法const t1={a:1};Object.assign(t1,{b:2},{c:3});console.log(t1);//generatorfunction* bear(){ ???console.log(‘熊大‘); ???console.log(‘熊二‘);}bear().next();//reactReactDOM.render( ???<h2>这是JSX语法</h2>, ???document.getElementById(‘box‘));webpack.config.js内容如下：
const path=require(‘path‘);const HtmlWebpackPlugin=require(‘html-webpack-plugin‘);const MiniCssExtractPlugin=require(‘mini-css-extract-plugin‘);module.exports={ ???entry:‘./src/js/index.js‘, ???output:{ ???????path:path.resolve(__dirname,‘dist‘), ???????filename:‘js/index.js‘ ???}, ???plugins:[ ???????new HtmlWebpackPlugin({ ???????????title:‘陈学辉‘, ???????????template:‘./src/template.html‘, ???????????filename:‘index.html‘ ???????}), ???????new MiniCssExtractPlugin({ ???????????filename:‘css/index.css‘ ???????}), ???], ???devServer:{ ???????host:‘localhost‘, ???????port:1573, ???????open:true ???}, ???module:{ ???????rules:[ ???????????{ ???????????????test:/\.css$/, ???????????????use:[ ???????????????????{ ???????????????????????loader:MiniCssExtractPlugin.loader, ???????????????????????options:{ ???????????????????????????publicPath:‘../‘ ???????????????????????} ???????????????????}, ???????????????????‘css-loader‘, ???????????????] ???????????}, ???????????{ ???????????????test:/\.less$/, ???????????????use:[ ??//把less编译到css文件里 ???????????????????{ ???????????????????????loader:MiniCssExtractPlugin.loader, ???????????????????????options:{ ???????????????????????????publicPath:‘../‘ ???????????????????????} ???????????????????}, ???????????????????‘css-loader‘, ??//注意顺序 ???????????????????‘less-loader‘ ???????????????] ???????????}, ???????????{ ???????????????test:/\.(jpg|png|gif)$/, ???????????????use:[ ???????????????????{ ???????????????????????loader:‘url-loader‘, ???????????????????????options:{ ???????????????????????????limit:5 * 1024, ???????????????????????????outputPath:‘images‘ ???????????????????????} ???????????????????} ???????????????] ???????????}, ???????????{ ???????????????test:/\.js$/, ???????????????use:[ ???????????????????{ ???????????????????????loader:‘babel-loader‘, ???????????????????????options:{ ??//env针对的是ES的版本，它会自动选择。react针对的就是react ???????????????????????????presets: [‘env‘,‘react‘] ???????????????????????} ???????????????????} ???????????????], ???????????????//exclude: /node_modules/, ?//不去检查node_modules里的内容，那里的js太多了，会非常慢 ???????????????include:path.resolve(__dirname,‘src/js‘), ??//直接规定查找的范围 ???????????} ???????] ???}}执行命令npm run build后，把文件在IE里面打开看看，也是Ok的~
注意：如果用过babel的同学可能知道还需要创建一个.babelrc的文件，那里面还要写一个对象。但是在这里却不需要，是因为babel-loader的options里已经把内容添上了，就不需要创建文件了
源码下载：https://pan.baidu.com/s/1KxoQJGgJdQqtzS_IGNrmuA
下一篇：Webpack 4.X 从入门到精通 - splitChunks（五）
Webpack 4.X 从入门到精通 - loader（五）
原文地址：http://blog.51cto.com/13258913/2162220