jiaojiu/node_modules/babel-preset-env/README.md

Now that babel-preset-env has stabilized, it has been moved into the main Babel mono-repo and this repo has been archived.

The move makes it much easier to release and develop in sync with the rest of Babel!

This repo will be made read-only, as all of the issues/labels have been moved over as well. Please report any bugs and open pull requests over on the main mono-repo.

---

babel-preset-env

A Babel preset that compiles ES2015+ down to ES5 by automatically determining the Babel plugins and polyfills you need based on your targeted browser or runtime environments.

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

Without any configuration options, babel-preset-env behaves exactly the same as babel-preset-latest (or babel-preset-es2015, babel-preset-es2016, and babel-preset-es2017 together).

However, we don't recommend using preset-env this way because it doesn't take advantage of it's greater capabilities of targeting specific browsers.

{
  "presets": ["env"]
}

You can also configure it to only include the polyfills and transforms needed for the browsers you support. Compiling only what's needed can make your bundles smaller and your life easier.

This example only includes the polyfills and code transforms needed for coverage of users > 0.25%, ignoring Internet Explorer 11 and Opera Mini.. We use browserslist to parse this information, so you can use any valid query format supported by browserslist.

{
  "presets": [
    ["env", {
      "targets": {
        // The % refers to the global coverage of users from browserslist
        "browsers": [ ">0.25%", "not ie 11", "not op_mini all"]
      }
    }]
  ]
}

You can also target individual versions of browsers instead of using a query with "targets": { "chrome": "52" }.

Similarly, if you're targeting Node.js instead of the browser, you can configure babel-preset-env to only include the polyfills and transforms necessary for a particular version:

{
  "presets": [
    ["env", {
      "targets": {
        "node": "6.10"
      }
    }]
  ]
}

For convenience, you can use "node": "current" to only include the necessary polyfills and transforms for the Node.js version that you use to run Babel:

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

Check out the many options (especially useBuiltIns to polyfill less)!

• 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 6, 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 | 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.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

true

When using uglify-js to minify your code, you may run into syntax errors when targeting later browsers since uglify-js does not support any ES2015+ syntax.

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

Uglify has support for ES2015 syntax via uglify-es. If you are using syntax unsupported by uglify-es, we recommend using babel-minify.

Note: This option is deprecated in 2.x and replaced with a forceAllTransforms option.

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" | 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 [].

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

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.

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

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

boolean, defaults to false.

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

NOTE: This does not currently polyfill experimental/stage-x built-ins like the regular "babel-polyfill" does. This will only work with npm >= 3 (which should be used with Babel 6 anyway)

npm install babel-polyfill --save

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.

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.

In

import "babel-polyfill";

Out (different based on environment)

import "core-js/modules/es7.string.pad-start";
import "core-js/modules/es7.string.pad-end";
import "core-js/modules/web.timers";
import "core-js/modules/web.immediate";
import "core-js/modules/web.dom.iterable";

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

npm install core-js --save

---

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
• ?