Edit

babel.config.js

Lookup behavior

The lookup behavior is direct as Babel will only look for a babel.config.js in the root directory (where your package.json is by default). If configuration is needed, see specifying the root folder.

Merging behavior

Unlike other formats (see the configuration guide), Babel won't try to merge configurations.

project
├── package.json
├── babel.config.js
├── node_modules
│   └── depA
│       ├── .babelrc // ignored
│       └── index.js // babel.config.js will be used
└── src
    ├── .babelrc // ignored
    └── index.js // babel.config.js will be used

Note: the configuration defined in babel.config.js is applied to your entire project.

Dynamic configuration

Since it's just a regular JavaScript file, you can write any arbitrary logic or use the Nodejs API:

const pkg = require("./package.json");

module.exports = function () {
  const plugins = [ ... ];

  if (pkg.authors.includes("Fieri")) {
    plugins.push(...);
  }

  return { plugins };
}

Environment

A conditional configuration is useful when you want to use certain plugins/presets based on the current environment, for example:

module.exports = function () {
  const plugins = [ ... ];

  if (process.env["REMOVE_DEBUG"] === 1) {
    plugins.push("babel-plugin-remove-console-debug");
  }

  return { plugins };
}

You can invoke it using the CLI:

REMOVE_DEBUG=1 babel file.js

API

Babel will pass an object as first argument, called api here.

module.exports = function (api) {
  const babelEnv = api.env();

  return { ... };
}

api.env()

Returns Babel's environment, you can configure it using the BABEL_ENV environment variable.

api.version

Returns Babel's core version.

api.cache()

// TODO

Specifying the root folder

You can pass a root in the Babel configuration, see @Babel/core's documentation.

Extending other .babelrc

To allow people to opt into .babelrc usage, potentially for local development; you can specify a list of .babelrc to use.

For example using babel-loader:

test: /\.js$/,
loader: "babel-loader",
options: {
  babelrc: [
    ".",
    "../some-linked-package"
  ]
}