How to use the optional runtime transformer.

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 runtime optional transformer 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. Built-ins such as Promise, Set and Map are aliased to core-js so you can use them seamlessly without having to require a globally polluting polyfill. This is fantastic for libraries as you can write your code without the cognitive overhead of worrying about the environment in which your code will be run.

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

External package required

The package babel-runtime is required for this transformer. Run npm install babel-runtime --save-dev to add it to your current node/webpack/browserify project. babel-runtime does not support AMD module loaders like RequireJS.


require("babel").transform("code", { optional: ["runtime"] });
$ babel --optional runtime script.js

Technical details

The runtime optional transformer 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.

Regenerator aliasing

Whenever you use a generator function or async function:

function* foo() {


async function bar() {


the following is generated:

"use strict";

var foo = regeneratorRuntime.mark(function foo() {

function bar() {
  return regeneratorRuntime.async(function bar$(context$1$0) {
  }, null, this);

This isn't ideal as then you have to include the regenerator runtime which pollutes the global scope.

Instead what the runtime transformer does it compile that to:

"use strict";

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

var foo = _regeneratorRuntime.mark(function foo() {

function bar() {
  return _regeneratorRuntime.async(function bar$(context$1$0) {
  }, null, 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 _core = require("babel-runtime/core-js");

var sym = _core.Symbol();

var promise = new _core.Promise();


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:

import foo from "bar";

usually turns into:

"use strict";

var _interopRequire = function (obj) {
  return obj && obj.__esModule ? obj["default"] : obj;

var foo = _interopRequire(require("bar"));

the runtime transformer however turns this into:

"use strict";

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

var foo = _babelHelpers.interopRequire(require("bar"));