/Babel 6

Runtime transform

Externalise references to helpers and builtins, automatically polyfilling your code without polluting globals

This plugin is recommended in a library/tool.

NOTE: Instance methods such as "foobar".includes("foo") will not work since that would require modification of existing built-ins (Use babel-polyfill for that).


Babel uses very small helpers for common functions such as _extend. By default this will be added to every file that requires it. This duplication is sometimes unnecessary, especially when your application is spread out over multiple files.

This is where the transform-runtime plugin comes in: all of the helpers will reference the module babel-runtime to avoid duplication across your compiled output. The runtime will be compiled into your build.

Another purpose of this transformer is to create a sandboxed environment for your code. If you use babel-polyfill and the built-ins it provides such as Promise, Set and Map, those will pollute the global scope. While this might be ok for an app or a command line tool, it becomes a problem if your code is a library which you intend to publish for others to use or if you can’t exactly control the environment in which your code will run.

The transformer will alias these built-ins to core-js so you can use them seamlessly without having to require the polyfill.

See the technical details section for more information on how this works and the types of transformations that occur.


NOTE - Production vs. development dependencies

In most cases, you should install babel-plugin-transform-runtime as a development dependency (with --save-dev).

npm install --save-dev babel-plugin-transform-runtime

and babel-runtime as a production dependency (with --save).

npm install --save babel-runtime

The transformation plugin is typically used only in development, but the runtime itself will be depended on by your deployed/published code. See the examples below for more details.


Add the following line to your .babelrc file:

Without options:

  "plugins": ["transform-runtime"]

With options:

  "plugins": [
    ["transform-runtime", {
      "helpers": false,
      "polyfill": false,
      "regenerator": true,
      "moduleName": "babel-runtime"


babel --plugins transform-runtime script.js

Via Node API

require("babel-core").transform("code", {
  plugins: ["transform-runtime"]



boolean, defaults to true.

Toggles whether or not inlined Babel helpers (classCallCheck, extends, etc.) are replaced with calls to moduleName.

For more information, see Helper aliasing.


boolean, defaults to true.

Toggles whether or not new built-ins (Promise, Set, Map, etc.) are transformed to use a non-global polluting polyfill.

For more information, see core-js aliasing.


boolean, defaults to true.

Toggles whether or not generator functions are transformed to use a regenerator runtime that does not pollute the global scope.

For more information, see Regenerator aliasing.


string, defaults to "babel-runtime".

Sets the name/path of the module used when importing helpers.


  "moduleName": "flavortown/runtime"
import extends from 'flavortown/runtime/helpers/extends';

Technical details

The runtime transformer plugin does three things:

  • Automatically requires babel-runtime/regenerator when you use generators/async functions.
  • Automatically requires babel-runtime/core-js and maps ES6 static methods and built-ins.
  • Removes the inline Babel helpers and uses the module babel-runtime/helpers instead.

What does this actually mean though? Basically, you can use built-ins such as Promise, Set, Symbol, etc., as well use all the Babel features that require a polyfill seamlessly, without global pollution, making it extremely suitable for libraries.

Make sure you include babel-runtime as a dependency.

Regenerator aliasing

Whenever you use a generator function or async function:

function* foo() {


the following is generated:

"use strict";

var _marked = [foo].map(regeneratorRuntime.mark);

function foo() {
  return regeneratorRuntime.wrap(function foo$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
        case "end":
          return _context.stop();
  }, _marked[0], this);

This isn’t ideal since it relies on the regenerator runtime being included, which pollutes the global scope.

With the runtime transformer, however, it is compiled to:

"use strict";

var _regenerator = require("babel-runtime/regenerator");

var _regenerator2 = _interopRequireDefault(_regenerator);

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

var _marked = [foo].map(_regenerator2.default.mark);

function foo() {
  return _regenerator2.default.wrap(function foo$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
        case "end":
          return _context.stop();
  }, _marked[0], this);

This means that you can use the regenerator runtime without polluting your current environment.

core-js aliasing

Sometimes you may want to use new built-ins such as Map, Set, Promise etc. Your only way to use these is usually to include a globally polluting polyfill.

What the runtime transformer does is transform the following:

var sym = Symbol();

var promise = new Promise;


into the following:

"use strict";

var _getIterator2 = require("babel-runtime/core-js/get-iterator");

var _getIterator3 = _interopRequireDefault(_getIterator2);

var _promise = require("babel-runtime/core-js/promise");

var _promise2 = _interopRequireDefault(_promise);

var _symbol = require("babel-runtime/core-js/symbol");

var _symbol2 = _interopRequireDefault(_symbol);

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

var sym = (0, _symbol2.default)();

var promise = new _promise2.default();

console.log((0, _getIterator3.default)(arr));

This means is that you can seamlessly use these native built-ins and static methods without worrying about where they come from.

NOTE: Instance methods such as "foobar".includes("foo") will not work.

Helper aliasing

Usually Babel will place helpers at the top of your file to do common tasks to avoid duplicating the code around in the current file. Sometimes these helpers can get a little bulky and add unnecessary duplication across files. The runtime transformer replaces all the helper calls to a module.

That means that the following code:

class Person {

usually turns into:

"use strict";

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Person = function Person() {
  _classCallCheck(this, Person);

the runtime transformer however turns this into:

"use strict";

var _classCallCheck2 = require("babel-runtime/helpers/classCallCheck");

var _classCallCheck3 = _interopRequireDefault(_classCallCheck2);

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

var Person = function Person() {
  (0, _classCallCheck3.default)(this, Person);

© 2018 Sebastian McKenzie
Licensed under the MIT License.