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@babel/plugin-transform-runtime

A plugin that enables the re-use of Babel's injected helper code to save on codesize.

Installation​

Install it as development dependency.

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

and @babel/runtime as a production dependency (since it's for the "runtime").

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 code. See the examples below for more details.

danger

When this plugin is enabled, the useBuiltIns option in @babel/preset-env must not be set. Otherwise, this plugin may not able to completely sandbox the environment.

Why?​

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 @babel/plugin-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.

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

Usage​

Without options:

babel.config.json
{
  "plugins": ["@babel/plugin-transform-runtime"]
}

With options (and their defaults):

babel.config.json
{
  "plugins": [
    [
      "@babel/plugin-transform-runtime",
      {
        "absoluteRuntime": false,
        "corejs": false,
        "helpers": true,
        "regenerator": true,
        "version": "7.0.0-beta.0"
      }
    ]
  ]
}

The plugin defaults to assuming that all polyfillable APIs will be provided by the user. Otherwise the corejs option needs to be specified.

Via CLI​

Shell
babel --plugins @babel/plugin-transform-runtime script.js

Via Node API​

JavaScript
require("@babel/core").transformSync("code", {
  plugins: ["@babel/plugin-transform-runtime"],
});

Options​

absoluteRuntime​

boolean or string, defaults to false.

This allows users to run transform-runtime broadly across a whole project. By default, transform-runtime imports from @babel/runtime/foo directly, but that only works if @babel/runtime is in the node_modules of the file that is being compiled. This can be problematic for nested node_modules, npm-linked modules, or CLIs that reside outside the user's project, among other cases. To avoid worrying about how the runtime module's location is resolved, this allows users to resolve the runtime once up front, and then insert absolute paths to the runtime into the output code.

Using absolute paths is not desirable if files are compiled for use at a later time, but in contexts where a file is compiled and then immediately consumed, they can be quite helpful.

tip

You can read more about configuring plugin options here

moduleName​

History
VersionChanges
v7.24.0Added moduleName option

string, defaults to @babel/runtime.

This option controls which package of helpers @babel/plugin-transform-runtime will use when injecting imports. It uses the following priority:

  • moduleName option, if specified
  • Helpers module suggested by any babel-plugin-polyfill-* plugin
    • babel-plugin-polyfill-corejs3 suggests @babel/runtime-corejs3
    • babel-plugin-polyfill-corejs2 suggests @babel/runtime-corejs2
  • Fallback to @babel/runtime

Note that specifying the corejs option will internally enable the corresponding babel-plugin-polyfill-corejs* plugin, thus it has an effect on the final module name.

version​

By default transform-runtime assumes that @babel/runtime@8.0.0 is installed. If you have later versions of @babel/runtime (or their corejs counterparts e.g. @babel/runtime-corejs3) installed or listed as a dependency, transform-runtime can use more advanced features.

For example if you depend on @babel/runtime@^8.1.0 you can transpile your code with

babel.config.json
{
  "plugins": [
    ["@babel/plugin-transform-runtime", {
      "version": "^8.1.0"
    }]
  ]
}

which results in a smaller bundle size.

Technical details​

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:

JavaScript
class Person {}

usually turns into:

JavaScript
"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:

JavaScript
"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);
};

Removed options​

The following options where removed in Babel 8.0.0:

  • corejs
  • helpers
  • regenerator

The following options where removed in Babel 7.0.0:

  • useBuiltIns
  • polyfill