Configure Babel

Babel can be configured! Many other tools have similar configs: ESLint (.eslintrc), Prettier (.prettierrc).

All Babel API options are allowed. However, if the option requires JavaScript, you may want to use a JavaScript configuration file.

What’s your use case?

• You are using a monorepo?
• You want to compile node_modules?

babel.config.json is for you!

• You have a configuration that only applies to a single part of your project?

.babelrc.json is for you!

• Guy Fieri is your hero?

We recommend using the babel.config.json format. Babel itself is using it.

babel.config.json

Create a file called babel.config.json with the following content at the root of your project (where the package.json is).

{
  "presets": [...],
  "plugins": [...]
}

module.exports = function (api) {
  api.cache(true);
  const presets = [ ... ];
  const plugins = [ ... ];
  return {
    presets,
    plugins
  };
}

Check out the babel.config.json documentation to see more configuration options.

.babelrc.json

Create a file called .babelrc.json with the following content in your project.

{
  "presets": [...],
  "plugins": [...]
}

Check out the .babelrc documentation to see more configuration options.

package.json

Alternatively, you can choose to specify your .babelrc.json config from within package.json using the babel key like so:

{
  "name": "my-package",
  "version": "1.0.0",
  "babel": {
    "presets": [ ... ],
    "plugins": [ ... ],
  }
}

JavaScript configuration files

You can also write babel.config.json and .babelrc.json files using JavaScript:

const presets = [ ... ];
const plugins = [ ... ];
module.exports = { presets, plugins };

You are allowed to access any Node.js APIs, for example a dynamic configuration based on the process environment:

const presets = [ ... ];
const plugins = [ ... ];
if (process.env["ENV"] === "prod") {
  plugins.push(...);
}
module.exports = { presets, plugins };

You can read more about JavaScript configuration files in the dedicated documentation

Using the CLI (@babel/cli)

babel --plugins @babel/plugin-transform-arrow-functions script.js

Check out the babel-cli documentation to see more configuration options.

Using the API (@babel/core)

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

Check out the babel-core documentation to see more configuration options.

Print effective configs

You can tell Babel to print effective configs on a given input path

# *nix or WSL
BABEL_SHOW_CONFIG_FOR=./src/myComponent.jsx npm start

$env:BABEL_SHOW_CONFIG_FOR = ".\src\myComponent.jsx"; npm start

BABEL_SHOW_CONFIG_FOR accepts both absolute and relative file paths. If it is a relative path, it will be resolved from cwd.

Once Babel processes the input file specified by BABEL_SHOW_CONFIG_FOR, Babel will print effective configs to the console. Here is an example output:

Babel configs on "/path/to/cwd/src/index.js" (ascending priority):
config /path/to/cwd/babel.config.json
{
  "sourceType": "script",
  "plugins": [
    "@foo/babel-plugin-1"
  ],
  "extends": "./my-extended.js"
}
config /path/to/cwd/babel.config.json .env["test"]
{
  "plugins": [
    [
      "@foo/babel-plugin-3",
      {
        "noDocumentAll": true
      },
    ]
  ]
}
config /path/to/cwd/babel.config.json .overrides[0]
{
  "test": "src/index.js",
  "sourceMaps": true
}
config /path/to/cwd/.babelrc
{}
programmatic options from @babel/cli
{
  "sourceFileName": "./src/index.js",
  "presets": [
    "@babel/preset-env"
  ],
  "configFile": "./my-config.js",
  "caller": {
    "name": "@babel/cli"
  },
  "filename": "./src/index.js"
}

Babel will print effective config sources ordered by ascending priority. Using the example above, the priority is:

babel.config.json < .babelrc < programmatic options from @babel/cli

In other words, babel.config.json is overwritten by .babelrc, and .babelrc is overwritten by programmatic options.

For each config source, Babel prints applicable config items (e.g. overrides and env) in the order of ascending priority. Generally each config sources has at least one config item — the root content of configs. If you have configured overrides or env, Babel will not print them in the root, but will instead output a separate config item titled as .overrides[index], where index is the position of the item. This helps determine whether the item is effective on the input and which configs it will override.

If your input is ignored by ignore or only, Babel will print that this file is ignored.

How Babel merges config items

Babel’s configuration merging is relatively straightforward. Options will overwrite existing options when they are present and their value is not undefined. There are, however, a few special cases:

• For assumptions, parserOpts and generatorOpts, objects are merged, rather than replaced.
• For plugins and presets, they are replaced based on the identity of the plugin/preset object/function itself combined with the name of the entry.

Option (except plugin/preset) merging

As an example, consider a config with:

{
  sourceType: "script",
  assumptions: {
    setClassFields: true,
    iterableIsArray: false
  },
  env: {
    test: {
      sourceType: "module",
      assumptions: {
        iterableIsArray: true,
      },
    }
  }
};

When NODE_ENV is test, the sourceType option will be replaced and the assumptions option will be merged. The effective config is:

{
  sourceType: "module", // sourceType: "script" is overwritten
  assumptions: {
    setClassFields: true,
    iterableIsArray: true, // assumptions are merged by Object.assign
  },
}

Plugin/Preset merging

As an example, consider a config with:

plugins: [
  './other',
  ['./plug', { thing: true, field1: true }]
],
overrides: [{
  plugins: [
    ['./plug', { thing: false, field2: true }],
  ]
}]

The overrides item will be merged on top of the top-level options. Importantly, the plugins array as a whole doesn’t just replace the top-level one. The merging logic will see that "./plug" is the same plugin in both cases, and { thing: false, field2: true } will replace the original options, resulting in a config as

plugins: [
  './other',
  ['./plug', { thing: false, field2: true }],
],

Since merging is based on identity + name, it is considered an error to use the same plugin with the same name twice in the same plugins/presets array. For example

plugins: ["./plug", "./plug"];

is considered an error, because it’s identical to plugins: ['./plug']. Additionally, even

plugins: [["./plug", { one: true }], ["./plug", { two: true }]];

is considered an error, because the second one would just always replace the first one.

If you actually do want to instantiate two separate instances of a plugin, you must assign each one a name to disambiguate them. For example:

plugins: [
  ["./plug", { one: true }, "first-instance-name"],
  ["./plug", { two: true }, "second-instance-name"],
];

because each instance has been given a unique name and thus a unique identity.