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@babel/register

One of the ways you can use Babel is through the require hook. The require hook will bind itself to node's require and automatically compile files on the fly. This is equivalent to CoffeeScript's coffee-script/register.

Install​

npm install @babel/core @babel/register --save-dev

Usage​

JavaScript
require("@babel/register");

All subsequent files required by node with the extensions .es6, .es, .jsx, .mjs, and .js will be transformed by Babel.

Polyfill not included

You must include the polyfill separately when using features that require it, like generators.

Ignores node_modules by default​

NOTE: By default all requires to node_modules will be ignored. You can override this by passing an ignore regex via:

JavaScript
require("@babel/register")({
  // This will override `node_modules` ignoring - you can alternatively pass
  // an array of strings to be explicitly matched or a regex / glob
  ignore: [],
});

Specifying options​

JavaScript
require("@babel/register")({
  // Array of ignore conditions, either a regex or a function. (Optional)
  // File paths that match any condition are not compiled.
  ignore: [
    // When a file path matches this regex then it is **not** compiled
    /regex/,

    // The file's path is also passed to any ignore functions. It will
    // **not** be compiled if `true` is returned.
    function(filepath) {
      return filepath !== "/path/to/es6-file.js";
    },
  ],

  // Array of accept conditions, either a regex or a function. (Optional)
  // File paths that match all conditions are compiled.
  only: [
    // File paths that **don't** match this regex are not compiled
    /my_es6_folder/,

    // File paths that **do not** return true are not compiled
    function(filepath) {
      return filepath === "/path/to/es6-file.js";
    },
  ],

  // Setting this will remove the currently hooked extensions of `.es6`, `.es`, `.jsx`, `.mjs`
  // and .js so you'll have to add them back if you want them to be used again.
  extensions: [".es6", ".es", ".jsx", ".js", ".mjs"],

  // Setting this to false will disable the cache.
  cache: true,
});

You can pass in all other options as well, including plugins and presets. Note that config files will also be loaded and the programmatic config will be merged over top of the file config options. @babel/register does not support ignore and only in config files.

Environment variables​

By default @babel/node cli and @babel/register will save to a json cache in your temporary directory.

This will heavily improve with the startup and compilation of your files. There are however scenarios where you want to change this behaviour and there are environment variables exposed to allow you to do this.

BABEL_CACHE_PATH​

Specify a different cache location.

Shell
BABEL_CACHE_PATH=/foo/my-cache.json babel-node script.js

BABEL_DISABLE_CACHE​

Disable the cache.

Shell
BABEL_DISABLE_CACHE=1 babel-node script.js

Compiling plugins and presets on the fly​

@babel/register uses Node's require() hook system to compile files on the fly when they are loaded. While this is quite helpful overall, it means that there can be confusing cases where code within a require() hook causes more calls to require, causing a dependency cycle. In Babel's case for instance, this could mean that in the process of Babel trying to compile a user's file, Babel could end up trying to compile itself as it is loading.

To avoid this problem, this module explicitly disallows re-entrant compilation, e.g. Babel's own compilation logic explicitly cannot trigger further compilation of any other files on the fly. The downside of this is that if you want to define a plugin or preset that is itself live-compiled, the process is complicated.

The crux of it is that your own code needs to load the plugin/preset first. Assuming the plugin/preset loads all of its dependencies up front, what you'll want to do is:

require("@babel/register")({
  // ...
});

require("./my-plugin");

Because it is your own code that triggered the load, and not the logic within @babel/register itself, this should successfully compile any plugin/preset that loads synchronously.

Experimental Babel 8 implementation​

You can also test the new experimental implementation that will be enabled by default in Babel 8, using

JavaScript
require("@babel/register/experimental-worker");

It internally runs Babel asynchronously, so it's compatible with .mjs configuration files. You can already use it as a replacement of @babel/register with a few caveats:

  • If you programmatically specify @babel/register options (using require("@babel/register")({ /* ... options */ })), you must make sure that they are serializable. This means that you cannot pass plugin functions defined inline, but you must move them to a separate ./my-plugin.js file or to a babel.config.js file.
  • The new implementation is still experimental: it should have the same features as the existing one, but there may be some new bugs and regressions.

Note: @babel/register does not support compiling native Node.js ES modules on the fly, since currently there is no stable API for intercepting ES modules loading.