为什么我们需要使用 import 'babel-polyfill'；在反应组件中？

Question

我想知道如果我们使用 babel loader + 所有预设，为什么我们需要在我们的组件中包含 babel-polyfill？我只是认为 babel-loader 应该自己完成所有工作。

示例取自https://github.com/reactjs/redux/tree/master/examples

我要问的是：

import 'babel-polyfill';
import React from 'react';
import { render } from 'react-dom';
import App from './containers/App';

这是包装示例：

{
  "name": "redux-shopping-cart-example",
  "version": "0.0.0",
  "description": "Redux shopping-cart example",
  "scripts": {
    "start": "node server.js",
    "test": "cross-env NODE_ENV=test mocha --recursive --compilers js:babel-register",
    "test:watch": "npm test -- --watch"
  },
  "repository": {
    "type": "git",
    "url": "https://github.com/reactjs/redux.git"
  },
  "license": "MIT",
  "bugs": {
    "url": "https://github.com/reactjs/redux/issues"
  },
  "homepage": "http://redux.js.org",
  "dependencies": {
    "babel-polyfill": "^6.3.14",
    "react": "^0.14.7",
    "react-dom": "^0.14.7",
    "react-redux": "^4.2.1",
    "redux": "^3.2.1",
    "redux-thunk": "^1.0.3"
  },
  "devDependencies": {
    "babel-core": "^6.3.15",
    "babel-loader": "^6.2.0",
    "babel-preset-es2015": "^6.3.13",
    "babel-preset-react": "^6.3.13",
    "babel-preset-react-hmre": "^1.1.1",
    "cross-env": "^1.0.7",
    "enzyme": "^2.0.0",
    "express": "^4.13.3",
    "json-loader": "^0.5.3",
    "react-addons-test-utils": "^0.14.7",
    "redux-logger": "^2.0.1",
    "mocha": "^2.2.5",
    "node-libs-browser": "^0.5.2",
    "webpack": "^1.9.11",
    "webpack-dev-middleware": "^1.2.0",
    "webpack-hot-middleware": "^2.9.1"
  }
}

这是取自https://github.com/reactjs/redux/tree/master/examples的 webpack 配置示例

var path = require('path')
var webpack = require('webpack')

module.exports = {
  devtool: 'cheap-module-eval-source-map',
  entry: [
    'webpack-hot-middleware/client',
    './index'
  ],
  output: {
    path: path.join(__dirname, 'dist'),
    filename: 'bundle.js',
    publicPath: '/static/'
  },
  plugins: [
    new webpack.optimize.OccurenceOrderPlugin(),
    new webpack.HotModuleReplacementPlugin()
  ],
  module: {
    loaders: [
      {
        test: /\.js$/,
        loaders: [ 'babel?presets[]=react,presets[]=es2015,presets[]=react-hmre' ],
        exclude: /node_modules/,
        include: __dirname
      },
      {
        test: /\.json$/,
        loaders: [ 'json' ],
        exclude: /node_modules/,
        include: __dirname
      }
    ]
  }
}

Answer 1

Babel 会将您的代码转换为浏览器可以理解的内容，但生成的代码使用的功能可能会或可能不会在每个浏览器中都有效。例如 Object.assign 并非所有浏览器都支持，因此 babel-polyfill 会填充这些漏洞。它只是一个 polyfill 的集合，你通常会包含它们以支持旧版浏览器。

考虑这段代码：

const foo = {
  name: 'Homer'
};
const bar = Object.assign({}, foo, {age: '?'});
console.log(Object.keys(foo), Object.keys(bar));

Babel 会将其转换为几乎相同的：

'use strict';
var foo = {
  name: 'Homer'
};
var bar = Object.assign({}, foo, { age: '?' });
console.log(Object.keys(foo), Object.keys(bar));

因为这是正常的老式 JS 语法。但是，这并不意味着使用的原生函数在所有浏览器中都实现了，所以我们需要包含 polyfill。