babel-preset-env

Babel preset for all environments.

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

// .babelrc
{
  "presets": [
    ["env", {
      "targets": {
        "chrome": 54
      }
    }]
  ]
}

• How it Works
• Install
• Usage
  • Options
  • Examples
• Caveats

How it Works

Determine environment support for ECMAScript features

#7 - 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.

Maintain a mapping between javascript features and babel plugins

Currently located at pluginFeatures.js.

This should be straightforward to do in most cases. There might be cases were 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.

#14 - 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).

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

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 browsers option like autoprefixer

#19 - Use browserslist to also queries like > 1%, last 2 versions.

Install

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

Usage

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

{
  "presets": ["env"]
}

Options

targets: { [string]: number }

Defaults to {}.

Takes an object of environment versions to support. Each target environment takes a number (you can specify a decimal like node: 6.5)

Example environments: "chrome, edge, firefox, safari, ie, ios, node".

The data for this is currently at: /data/plugins.json and being generated by /scripts/build-data.js using https://kangax.github.io/compat-table.

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)

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.

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. Can be false to not transform modules.

debug: boolean

Defaults to false console.log out the targets and plugins being used as well as the version specified in /data/plugins.json.

whitelist: Array<string>

Defaults to [] Enable a whitelist of plugins to always include.

Useful if there is a bug in a native implementation, or a combination of a non-supported feature + a supported one doesn't work.

Ex: Node 4 supports native classes but not spread.

Examples

// src
export class A {}

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

// ...

class A {}
exports.A = A;

// target chrome 52 with webpack 2/rollup and loose mode
{
  "presets": [
    ["env", {
      "targets": {
        "chrome": 52
      },
      "modules": false,
      "loose": true
    }]
  ]
}

// ...

export class A {}

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

// ...

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

Example with node: true or node: "current"

// process.versions.node -> 6.9.0
{
  "presets": [
    ["env", {
      "targets": {
        "node": "current"
      }
    }]
  ]
}

// ...

class A {}
exports.A = A;

Example with debug: true

Using targets: {
  "node": 6.5
}

Using plugins:

module: false
transform-exponentiation-operator {}
transform-async-to-generator {}
syntax-trailing-function-commas {}

Example with whitelist

// target chrome 52 with whitelist on arrow functions
{
  "presets": [
    ["env", {
      "targets": {
        "chrome": 52
      },
      "whitelist": ["transform-es2015-arrow-functions"]
    }]
  ]
}

Using plugins:

transform-exponentiation-operator {}
transform-async-to-generator {}
syntax-trailing-function-commas {}
transform-es2015-arrow-functions {}

Caveats

If you get a SyntaxError: Unexpected token ... error if using the object-rest-spread then make sure the plugin is at v6.19.0.