preset-env

---

code>@babel/preset-env</code is a smart preset that allows you to use the latest JavaScript without needing to micromanage which syntax transforms (and optionally, browser polyfills) are needed by your target environment(s). This both makes your life easier and JavaScript bundles smaller!

• Install
• How Does it Work?
• Browserslist Integration
• Options

Install

With npm:

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

Or yarn:

 sh
yarn add @babel/preset-env --dev

How Does it Work?

code>@babel/preset-env</code would not be possible if not for a number of awesome open-source projects, like browserslist, compat-table, and electron-to-chromium.

We leverage these data sources to maintain mappings of which version of our supported target environments gained support of a JavaScript syntax or browser feature, as well as a mapping of those syntaxes and features to Babel transform plugins and core-js polyfills.

It is important to note that @babel/preset-env does not support stage-x plugins.

code>@babel/preset-env</code takes any target environments you’ve specified and checks them against its mappings to compile a list of plugins and passes it to Babel.

Browserslist Integration

For browser- or Electron-based projects, we recommend using a .browserslistrc file to specify targets. You may already have this configuration file as it is used by many tools in the ecosystem, like autoprefixer, stylelint, eslint-plugin-compat and many others.

By default code>@babel/preset-env</code will use browserslist config sources unless either the targets or ignoreBrowserslistConfig options are set.

For example, to only include polyfills and code transforms needed for users whose browsers have >0.25% market share (ignoring browsers without security updates like IE 10 and BlackBerry):

Options

 json
{
  "presets": [
    [
      "@babel/preset-env",
      {
        "useBuiltIns": "entry"
      }
    ]
  ]
}

browserslist

> 0.25%
not dead

or

package.json

"browserslist": "> 0.25%, not dead"

Options

For more information on setting options for a preset, refer to the preset options documentation.

targets

string | Array<string> | { [string]: string }, defaults to {}.

Describes the environments you support/target for your project.

This can either be a browserslist-compatible query:

 json
{
  "targets": "> 0.25%, not dead"
}

Or an object of minimum environment versions to support:

 json
{
  "targets": {
    "chrome": "58",
    "ie": "11"
  }
}

Example environments: chrome, opera, edge, firefox, safari, ie, ios, android, node, electron.

Sidenote, if no targets are specified, code>@babel/preset-env</code will transform all ECMAScript 2015+ code by default.

We don’t recommend using preset-env this way because it doesn’t take advantage of its ability to target specific browsers.

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

targets.esmodules

boolean.

You may also target browsers supporting ES Modules (https://www.ecma-international.org/ecma-262/6.0/#sec-modules). When specifying this option, the browsers field will be ignored. You can use this approach in combination with <script type="module"></script> to conditionally serve smaller scripts to users (https://jakearchibald.com/2017/es-modules-in-browsers/#nomodule-for-backwards-compatibility).

Please note: when specifying the esmodules target, browsers targets will be ignored.

 json
{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": {
          "esmodules": true
        }
      }
    ]
  ]
}

targets.node

string | "current" | true.

If you want to compile against the current node version, you can specify "node": true or "node": "current", which would be the same as "node": process.versions.node.

targets.safari

string | "tp".

If you want to compile against the technology preview version of Safari, you can specify "safari": "tp".

targets.browsers

string | Array<string>.

A query to select browsers (ex: last 2 versions, > 5%, safari tp) using browserslist.

Note, browsers’ results are overridden by explicit items from targets.

Note: this will be removed in later version in favor of just setting "targets" to a query directly.

spec

boolean, defaults to false.

Enable more spec compliant, but potentially slower, transformations for any plugins in this preset that support them.

loose

boolean, defaults to false.

Enable "loose" transformations for any plugins in this preset that allow them.

modules

"amd" | "umd" | "systemjs" | "commonjs" | "cjs" | "auto" | false, defaults to "auto".

Enable transformation of ES6 module syntax to another module type.

Setting this to false will not transform modules.

Also note that cjs is just an alias for commonjs.

debug

boolean, defaults to false.

Outputs the targets/plugins used and the version specified in plugin data version to console.log.

include

Array<string|RegExp>, defaults to [].

An array of plugins to always include.

Valid options include any:

• Babel plugins – both with (@babel/plugin-transform-spread) and without prefix (plugin-transform-spread) are supported.
• Built-ins, such as es6.map, es6.set, or es6.object.assign.

Plugin names can be fully or partially specified (or using RegExp).

Acceptable inputs:

• Full name (string): "es6.math.sign"
• Partial name (string): "es6.math.*" (resolves to all plugins with es6.math prefix)
• RegExp Object: /^transform-.*$/ or new RegExp("^transform-modules-.*")

Note that the above . is the RegExp equivalent to match any character, and not the actual '.' character. Also note that to match any character . is used in RegExp as opposed to in glob format.

This option is useful if there is a bug in a native implementation, or a combination of a non-supported feature + a supported one doesn’t work.

For example, Node 4 supports native classes but not spread. If super is used with a spread argument, then the code>@babel/plugin-transform-classes</code transform needs to be included, as it is not possible to transpile a spread with super otherwise.

NOTE: The include and exclude options only work with the plugins included with this preset; so, for example, including @babel/plugin-proposal-do-expressions or excluding @babel/plugin-proposal-function-bind will throw errors. To use a plugin not included with this preset, add them to your "plugins" directly.

exclude

Array<string|RegExp>, defaults to [].

An array of plugins to always exclude/remove.

The possible options are the same as the include option.

This option is useful for "blacklisting" a transform like code>@babel/plugin-transform-regenerator</code if you don’t use generators and don’t want to include regeneratorRuntime (when using useBuiltIns) or for using another plugin like fast-async instead of Babel’s async-to-gen.

useBuiltIns

"usage" | "entry" | false, defaults to false.

This option adds direct references to the core-js module as bare imports. Thus core-js will be resolved relative to the file itself and needs to be accessible. You may need to specify core-js@2 as a top level dependency in your application if there isn’t a core-js dependency or there are multiple versions.

This option configures how code>@babel/preset-env</code handles polyfills.

useBuiltIns: 'entry'

NOTE: Only use require("@babel/polyfill"); once in your whole app. Multiple imports or requires of @babel/polyfill will throw an error since it can cause global collisions and other issues that are hard to trace. We recommend creating a single entry file that only contains the require statement.

This option enables a new plugin that replaces the statement import "@babel/polyfill" or code>require(&quot;@babel/polyfill&quot;)</code with individual requires for code>@babel/polyfill</code based on environment.

 sh
npm install @babel/polyfill --save

In

 js
import "@babel/polyfill";

Out (different based on environment)

 js
import "core-js/modules/es7.string.pad-start";
import "core-js/modules/es7.string.pad-end";

This will also work for core-js directly (import "core-js"; or require('core-js');)

useBuiltIns: 'usage' (experimental)

Adds specific imports for polyfills when they are used in each file. We take advantage of the fact that a bundler will load the same polyfill only once.

In

a.js

 js
var a = new Promise();

b.js

 js
var b = new Map();

Out (if environment doesn’t support it)

 js
import "core-js/modules/es6.promise";
var a = new Promise();

 js
import "core-js/modules/es6.map";
var b = new Map();

Out (if environment supports it)

 js
var a = new Promise();

 js
var b = new Map();

useBuiltIns: false

Don’t add polyfills automatically per file, or transform import "@babel/polyfill" to individual polyfills.

forceAllTransforms

boolean, defaults to false.

Example

With Babel 7’s Javascipt config file support, you can force all transforms to be run if env is set to production.

 js
module.exports = function(api) {
  return {
    presets: [
      [
        "@babel/preset-env",
        {
          targets: {
            chrome: 59,
            edge: 13,
            firefox: 50,
          },
          // for uglifyjs...
          forceAllTransforms: api.env("production"),
        },
      ],
    ],
  };
};

NOTE: targets.uglify is deprecated and will be removed in the next major in favor of this.

By default, this preset will run all the transforms needed for the targeted environment(s). Enable this option if you want to force running all transforms, which is useful if the output will be run through UglifyJS or an environment that only supports ES5.

NOTE: Uglify has a work-in-progress "Harmony" branch to address the lack of ES6 support, but it is not yet stable. You can follow its progress in UglifyJS2 issue #448. If you require an alternative minifier which does support ES6 syntax, we recommend using babel-minify.

configPath

string, defaults to process.cwd()

The starting point where the config search for browserslist will start, and ascend to the system root until found.

ignoreBrowserslistConfig

boolean, defaults to false

Toggles whether or not browserslist config sources are used, which includes searching for any browserslist files or referencing the browserslist key inside package.json. This is useful for projects that use a browserslist config for files that won’t be compiled with Babel.

shippedProposals

boolean, defaults to false

Toggles enabling support for builtin/feature proposals that have shipped in browsers. If your target environments have native support for a feature proposal, its matching parser syntax plugin is enabled instead of performing any transform. Note that this does not enable the same transformations as code>@babel/preset-stage-3</code, since proposals can continue to change before landing in browsers.

The following are currently supported:

Builtins

• es7.array.flat-map

Features

• None

← registerstage-0 →

来源：Js中文网 – 前端进阶资源教程
链接：https://www.javascriptc.com/docs/babel-manual/babel-preset-env