前言
前面我们写了几篇很长的文章去介绍babel源码、preset-env、runtime，
- babel源码解析一
- babel源码解析之（@babel/preset-env）
- babel源码解析之（@babel/plugin-transform-runtime）
在babel配置中我们可能用过@babel/polyfill、core-js、core-js-pure、@babel/runtime、@babel/runtime-corejs2、@babel/runtime-corejs3、@babel/plugin-transform-runtime、@babel/preset-env，当然这些都有出现在我们的文章中的，并且很详情的说了每个应用场景，所以强烈推荐小伙伴去看一下这几篇文章。
总结
好啦～ 我们今天就总结一下这些babel配置之前的联系，然后最后说一下在实战项目中的一些配置建议。
@babel/polyfill
As of Babel 7.4.0, this package has been deprecated in favor of directly including
core-js/stable(to polyfill ECMAScript features) andregenerator-runtime/runtime(needed to use transpiled generator functions):
也就是说在babel7.4.0之后是弃用掉的，然后现在由core-js替换，core-js可以用以下代码来替换之前的@babel/polyfill：
//import "@babel/polyfill"; //之前的写法
import "core-js/stable";
import "regenerator-runtime/runtime";
core-js&core-js-pure
core-js出现其实就是为了能让你代码中的一些api（比如：Array.prototype.include）能够运行，等于是在你的代码跟浏览器中加入了一个垫片，所以在babel7.4.0之后所有的polyfill操作都依赖core-js，core-js-pure是core-js的另外一个版本，可以不污染全局变量，比如我们的@babel/plugin-transform-runtime插件就是依赖的core-js-pure，core-js的更多用法跟介绍大家可以看官网https://github.com/zloirock/core-js
@babel/runtime
@babel/runtime、@babel/runtime-corejs2、@babel/runtime-corejs3这几个都叫“@babel/runtime”，只是每一个对应的实现不一样，都是提供给@babel/plugin-transform-runtime插件做依赖，@babel/plugin-transform-runtime插件会根据corejs的配置做不通的runtime依赖，具体用法大家可以参考之前的几篇文章。
@babel/plugin-transform-runtime 和 @babel/preset-env
@babel/preset-env包含了一些基本es语法转换的插件（箭头函数、类转换等等），同时还支持polyfill，有usage跟entry模式，但是preset-env添加polyfill会像之前使用@babel/polyfill一样，会污染全局变量。
@babel/plugin-transform-runtime主要是利用@babel/runtime提取了一些公共的babel帮助函数，同时也支持polyfill的添加，添加的polyfill都是以一个局部变量的形式引入，不会污染全局变量。
如果你做的是一个二方库，然后需要被别人依赖，那么建议使用@babel/plugin-transform-runtime来引入polyfill，因为你要尽可能的专注于做自己的事，而不是说去影响别人，语法转换可以使用preset-env预设，比如以下配置：
module.exports = {
presets: [
[
"@babel/preset-env",
]
],
plugins: [
[
"@babel/plugin-transform-runtime",
{
corejs: {version: 3, proposals: true},
helpers: true,
useESModules: true,
regenerator: true,
absoluteRuntime: "./node_modules"
}
]
]
};
如果你做的是一个普通的业务项目的话，可以用preset-env来转换语法和polyfill，然后再利用@babel/plugin-transform-runtime来引入helpers跟generator做到代码重复利用，比如以下配置：
module.exports = {
presets: [
[
"@babel/preset-env",
{
corejs: 3,
useBuiltIns: 'usage',
}
]
],
plugins: [
[
"@babel/plugin-transform-runtime",
{
corejs: false,
helpers: true,
useESModules: false,
regenerator: true,
absoluteRuntime: "./node_modules"
}
]
]
};
小伙伴思维也不要被某种方式定死，还是要根据自己项目需要灵活配置，找到自己项目最好的就可以了，说白了不管是runtime还是preset-env还是之前的@babel/polyfill，掌握原理后随意搭配都是可以的，比如我们接下来看一下vue-cli4.1.1中是怎么对babel配置的。
@vue/cli-service@^4.1.1
大家可以安装一下@vue/cli-service@^4.1.1：
npm install -D @vue/cli-service@^4.1.1
然后我们找到vue-cli中对babel的配置文件，
node_modules/@vue/babel-preset-app/index.js：
const path = require('path')
const defaultPolyfills = [
// promise polyfill alone doesn't work in IE,
// needs this as well. see: #1642
'es.array.iterator',
// this is required for webpack code splitting, vuex etc.
'es.promise',
// this is needed for object rest spread support in templates
// as vue-template-es2015-compiler 1.8+ compiles it to Object.assign() calls.
'es.object.assign',
// #2012 es6.promise replaces native Promise in FF and causes missing finally
'es.promise.finally'
]
function getPolyfills (targets, includes, { ignoreBrowserslistConfig, configPath }) {
const getTargets = require('@babel/helper-compilation-targets').default
const builtInTargets = getTargets(targets, { ignoreBrowserslistConfig, configPath })
// if no targets specified, include all default polyfills
if (!targets && !Object.keys(builtInTargets).length) {
return includes
}
const { list } = require('core-js-compat')({ targets: builtInTargets })
return includes.filter(item => list.includes(item))
}
module.exports = (context, options = {}) => {
const presets = []
const plugins = []
const defaultEntryFiles = JSON.parse(process.env.VUE_CLI_ENTRY_FILES || '[]')
// Though in the vue-cli repo, we only use the two envrionment variables
// for tests, users may have relied on them for some features,
// dropping them may break some projects.
// So in the following blocks we don't directly test the `NODE_ENV`.
// Rather, we turn it into the two commonly used feature flags.
if (process.env.NODE_ENV === 'test') {
// Both Jest & Mocha set NODE_ENV to 'test'.
// And both requires the `node` target.
process.env.VUE_CLI_BABEL_TARGET_NODE = 'true'
// Jest runs without bundling so it needs this.
// With the node target, tree shaking is not a necessity,
// so we set it for maximum compatibility.
process.env.VUE_CLI_BABEL_TRANSPILE_MODULES = 'true'
}
// JSX
if (options.jsx !== false) {
presets.push([require('@vue/babel-preset-jsx'), typeof options.jsx === 'object' ? options.jsx : {}])
}
const runtimePath = path.dirname(require.resolve('@babel/runtime/package.json'))
const runtimeVersion = require('@babel/runtime/package.json').version
const {
polyfills: userPolyfills,
loose = false,
debug = false,
useBuiltIns = 'usage',
modules = false,
bugfixes = true,
targets: rawTargets,
spec,
ignoreBrowserslistConfig = !!process.env.VUE_CLI_MODERN_BUILD,
configPath,
include,
exclude,
shippedProposals,
forceAllTransforms,
decoratorsBeforeExport,
decoratorsLegacy,
// entry file list
entryFiles = defaultEntryFiles,
// Undocumented option of @babel/plugin-transform-runtime.
// When enabled, an absolute path is used when importing a runtime helper after transforming.
// This ensures the transpiled file always use the runtime version required in this package.
// However, this may cause hash inconsistency if the project is moved to another directory.
// So here we allow user to explicit disable this option if hash consistency is a requirement
// and the runtime version is sure to be correct.
absoluteRuntime = runtimePath,
// https://babeljs.io/docs/en/babel-plugin-transform-runtime#version
// By default transform-runtime assumes that @babel/[email protected] is installed, which means helpers introduced later than 7.0.0-beta.0 will be inlined instead of imported.
// See https://github.com/babel/babel/issues/10261
// And https://github.com/facebook/docusaurus/pull/2111
version = runtimeVersion
} = options
// resolve targets
let targets
if (process.env.VUE_CLI_BABEL_TARGET_NODE) {
// running tests in Node.js
targets = { node: 'current' }
} else if (process.env.VUE_CLI_BUILD_TARGET === 'wc' || process.env.VUE_CLI_BUILD_TARGET === 'wc-async') {
// targeting browsers that at least support ES2015 classes
// https://github.com/babel/babel/blob/master/packages/babel-preset-env/data/plugins.json#L52-L61
targets = {
browsers: [
'Chrome >= 49',
'Firefox >= 45',
'Safari >= 10',
'Edge >= 13',
'iOS >= 10',
'Electron >= 0.36'
]
}
} else if (process.env.VUE_CLI_MODERN_BUILD) {
// targeting browsers that support