babel-preset-env

A Babel preset that can automatically determine the Babel plugins and polyfills you need based on your supported environments.

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

{
  "presets": [
    ["env", {
      "targets": {
        "browsers": ["last 2 versions", "safari >= 7"]
      }
    }]
  ]
}

• How it Works
• Install
• Usage
• Options
• Examples
• Caveats
• Other Cool Projects

How it Works

Determine environment support for ECMAScript features

Use external data such as compat-table to determine browser support. (We should create PRs there when necessary)

We can periodically run build-data.js which generates plugins.json.

Ref: #7

Maintain a mapping between JavaScript features and Babel plugins

Currently located at plugin-features.js.

This should be straightforward to do in most cases. There might be cases where plugins should be split up more or certain plugins aren't standalone enough (or impossible to do).

Support all plugins in Babel that are considered latest

Default behavior without options is the same as babel-preset-latest.

It won't include stage-x plugins. env will support all plugins in what we consider the latest version of Javascript (by matching what we do in babel-preset-latest).

Ref: #14

Determine the lowest common denominator of plugins to be included in the preset

If you are targeting IE 8 and Chrome 55 it will include all plugins required by IE 8 since you would need to support both still.

Support a target option "node": "current" to compile for the currently running node version.

For example, if you are building on Node 4, arrow functions won't be converted, but they will if you build on Node 0.12.

Support a browsers option like autoprefixer

Use browserslist to declare supported environments by performing queries like > 1%, last 2 versions.

Ref: #19

Install

With npm:

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

Or yarn:

yarn add babel-preset-env --dev

Usage

The default behavior without options runs all transforms (behaves the same as babel-preset-latest).

{
  "presets": ["env"]
}

Options

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

targets

{ [string]: number | string }, defaults to {}.

Takes an object of environment versions to support.

Each target environment takes a number or a string (we recommend using a string when specifying minor versions like node: "6.10").

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

The data for this is generated by running the build-data script which pulls in data from compat-table.

targets.node

number | "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": parseFloat(process.versions.node).

targets.browsers

Array<string> | string

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

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

targets.uglify

number | true

UglifyJS does not currently support any ES6 syntax, so if you are using Uglify to minify your code, targeting later browsers may cause Uglify to throw syntax errors.

To prevent these errors - specify the uglify option, which will enable all plugins and, as a result, fully compile your code to ES5. However, the useBuiltIns option will still work as before, and only include the polyfills that your target(s) need.

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 Babili.

loose

boolean, defaults to false.

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

modules

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

Enable transformation of ES6 module syntax to another module type.

Setting this to false will not transform modules.

debug

boolean, defaults to false.

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

include

Array<string>, defaults to [].

An array of plugins to always include.

Valid options include any:

• Babel plugins - both with (babel-plugin-transform-es2015-spread) and without prefix (transform-es2015-spread) are supported.

• Built-ins, such as map, set, or object.assign.

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 transform-es2015-classes transform needs to be included, as it is not possible to transpile a spread with super otherwise.

exclude

Array<string>, 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 transform-regenerator 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.

A way to apply babel-preset-env for polyfills (via babel-polyfill).

npm install babel-polyfill --save

useBuiltIns: 'usage'

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

var a = new Promise();

b.js

var b = new Map();

Out (if environment doesn't support it)

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

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

Out (if environment supports it)

var a = new Promise();

var b = new Map();

useBuiltIns: 'entry'

NOTE: Only use require("babel-polyfill"); once in your whole app. One option is to create a single entry file that only contains the require statement.

This option enables a new plugin that replaces the statement import "babel-polyfill" or require("babel-polyfill") with individual requires for babel-polyfill based on environment.

In

import "babel-polyfill";

Out (different based on environment)

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

useBuiltIns: false

Don't add polyfills automatically per file, or transform import "babel-polyfill" to individual polyfills.

Examples

Export with various targets

export class A {}

Target only Chrome 52

.babelrc

{
  "presets": [
    ["env", {
      "targets": {
        "chrome": 52
      }
    }]
  ]
}

Out

class A {}
exports.A = A;

Target Chrome 52 with webpack 2/rollup and loose mode

.babelrc

{
  "presets": [
    ["env", {
      "targets": {
        "chrome": 52
      },
      "modules": false,
      "loose": true
    }]
  ]
}

Out

export class A {}

Target specific browsers via browserslist

.babelrc

{
  "presets": [
    ["env", {
      "targets": {
        "chrome": 52,
        "browsers": ["last 2 versions", "safari 7"]
      }
    }]
  ]
}

Out

export var A = function A() {
  _classCallCheck(this, A);
};

Target latest node via node: true or node: "current"

.babelrc

{
  "presets": [
    ["env", {
      "targets": {
        "node": "current"
      }
    }]
  ]
}

Out

class A {}
exports.A = A;

Show debug output

.babelrc

{
  "presets": [
    [ "env", {
      "targets": {
        "safari": 10
      },
      "modules": false,
      "useBuiltIns": true,
      "debug": true
    }]
  ]
}

stdout

Using targets:
{
  "safari": 10
}

Modules transform: false

Using plugins:
  transform-exponentiation-operator {}
  transform-async-to-generator {}

Using polyfills:
  es7.object.values {}
  es7.object.entries {}
  es7.object.get-own-property-descriptors {}
  web.timers {}
  web.immediate {}
  web.dom.iterable {}

Include and exclude specific plugins/built-ins

always include arrow functions, explicitly exclude generators

{
  "presets": [
    ["env", {
      "targets": {
        "browsers": ["last 2 versions", "safari >= 7"]
      },
      "include": ["transform-es2015-arrow-functions", "es6.map"],
      "exclude": ["transform-regenerator", "es6.set"]
    }]
  ]
}

Caveats

If you get a SyntaxError: Unexpected token ... error when using the object-rest-spread transform then make sure the plugin has been updated to, at least, v6.19.0.

Other Cool Projects

• babel-preset-modern-browsers
• ?