webpack中使用babel

step one:

https://babeljs.io/setup    

Choose your tool (try CLI)

select webpack

Step two：

npm install --save-dev babel-loader @babel/core

module: {
  rules: [
    { test: /.js$/, exclude: /node_modules/, loader: "babel-loader" }
  ]
}

npm install @babel/preset-env --save-dev

In order to enable the preset you have to define it in your .babelrc file, like this:

{
  "presets": ["@babel/preset-env"]
}

如果不想添加.babelrc文件可以在webpack.config.js中直接这样写：

{
    test: /.js$/,
    exclude: /node_modules/,
    loader: "babel-loader",
    options: {
        presets: ["@babel/preset-env"]
    }
}

 Step three：

有些低版本的浏览器，没有内置新JavaScript语法，比如说promise、map等等

我们需要借助@babel/polyfill将这些语法添加到浏览器https://babeljs.io/docs/en/babel-polyfill

npm install --save @babel/polyfill

If you are using ES6's import syntax in your application's entry point, you should instead import the polyfill at the top of the entry point to ensure the polyfills are loaded first:

import "@babel/polyfill";

如果只想将程序中使用到的新语法添加到浏览器，而没有使用到的新语法不用添加可以这么配置：

{
    test: /.js$/,
    exclude: /node_modules/,
    loader: "babel-loader",
    options: {
       presets: [["@babel/preset-env", {
                  useBuiltIns: 'usage'  //如果配置了这个选项 业务代码里import "@babel/polyfill" 可以不用写 
　　　　　　　　　　}]] 

    } 

}

 注意：@babel/polyfill还有其它额外的配置下面是示例，详情请参考： https://babeljs.io/docs/en/usage

Creating a config file named babel.config.json in the root of your project with this content:

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": {
          "edge": "17",
          "firefox": "60",
          "chrome": "67",
          "safari": "11.1",
        },
        "useBuiltIns": "usage",
      }
    ]
  ]
}

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 [以上的配置在写组件库或类库代码时失效，因为@babel/polyfill是全局注入语法的，污染了全局环境]

 所以还有另外一种配置方案：https://babeljs.io/docs/en/babel-plugin-transform-runtime

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