Configuration File Types

Babel has two parallel config file formats, which can be used together, or independently.

• Project-wide configuration
• File-relative configuration
  • .babelrc (and .babelrc.js) files
  • package.json files with a "babel" key

Project-wide configuration

New in Babel 7.x, Babel has a concept of a "root" directory, which defaultsto the current working directory. For project-wide configuration, Babel will automatically searchfor a "babel.config.js" in this root directory. Alternatively, users can use an explicit"configFile" value to override the default config file search behavior.

Because project-wide config files are separated from the physical location of the configfile, they can be ideal for configuration that must apply broadly, even allowingplugins and presets to easily apply to files in node_modules or in symlinked packages,which were traditionally quite painful to configure in Babel 6.x.

The primary downside of this project-wide config is that, because it relies on the workingdirectory, it can be more painful to use in monorepos if the working directory is not the monorepo root.See the monorepo documentation for examples of how to use config files in that context.

Project-wide configs can also be disabled by setting "configFile" to false.

File-relative configuration

Babel loads .babelrc (and .babelrc.js / package.json#babel) files by searching up thedirectory structure starting from the "filename" being compiled (limited by the caveats below).This can be powerful because it allows you to create independent configurations for subsections ofa package. File-relative configurations are also merged over top ofproject-wide config values, making them potentially useful for specific overrides, though that canalso be accomplished through "overrides".

There are a few edge cases to consider when using a file-relative config:

• Searching will stop once a directory containing a package.json is found, so a relative configonly applies within a single package.
• The "filename" being compiled must be inside of"babelrcRoots" packages, or else searching will be skipped entirely.

These caveats mean that:

• .babelrc files only apply to files within their own package
• .babelrc files in packages that aren't Babel's 'root' are ignored unless you opt inwith "babelrcRoots".

See the monorepo documentation for more discussion on how to configure monorepos that have many packages.

File-relative configs can also be disabled by setting "babelrc" to false.

6.x vs 7.x .babelrc loading

Users coming from Babel 6.x will likely trip up on these two edge cases, which are new in Babel 7.x.These two restrictions were added to address common footguns in Babel 6.x:

• .babelrc files applied to node_modules dependencies, often unexpectedly.
• .babelrc files failed to apply to symlinked node_modules when people expected them to behave like normal dependencies.
• .babelrc files in node_modules dependencies would be detected, even though the plugins andpresets inside they were generally not installed, and may not even be valid in the version ofBabel compiling the file.

These cases will primarily cause issues for users with a monorepo structure, because if youhave

.babelrc
packages/
  mod1/
    package.json
    src/index.js
  mod2/
    package.json
    src/index.js

the config will now be entirely ignored, because it is across a package boundary.

One alternative would be to create a .babelrc in each sub-package that uses "extends" as

{ "extends": "../../.babelrc" }

Unfortunately, this approach can be a bit repetitive, and depending on how Babel is being used,could require setting "babelrcRoots".

Given that, it may be more desirable to rename the .babelrc to be aproject-wide "babel.config.js". As mentioned in the project-widesection above, this may then require explicitly setting "configFile"since Babel will not find the config file if the working directory isn't correct.

Monorepos

Monorepo-structured repositories usually contain many packages, which means that they frequentlyrun into the caveats mentioned in file-relative configurationand config file loading in general. This section is aimed at helping users understand howto approach monorepo configuration.

With monorepo setups, the core thing to understand is that Babel treats your working directoryas its logical "root", which causes problems if you want to run Babeltools within a specific sub-package without having Babel apply to the repo as a whole.

Separately, it is also important to decide if you want to use .babelrcfiles or just a central babel.config.js. .babelrcfiles are not required for subfolder-specific configuration like they were in Babel 6, sooften they are not needed in Babel 7, in favor of a babel.config.js.

Root babel.config.js files

The first step in any monorepo structure should be to create ababel.config.js file in repository root. Thisestablishes Babel's core concept of the base directory of your repository. Even if you want to use.babelrc files to configure each separate package, it is importantto have as a place for repo-level options.

You can often place all of your repo configuration in the root babel.config.js.With "overrides", you can easily specify configuration that only applies tocertain subfolders of your repository, which can often be easier to follow than creating many.babelrc files across the repo.

The first issue you'll likely run into is that by default, Babel expects to load babel.config.jsfiles from the directory set as its "root", which means that if you createa babel.config.js, but run Babel inside an individual package, e.g.

cd packages/some-package;
babel src -d dist

the "root" Babel is using in that context is not your monorepo root,and it won't be able to find the babel.config.js file.

If all of your build scripts run relative to your repository root, things should already work, but ifyou are running your Babel compilation process from within a subpackage, you need to tell Babel whereto look for the config. There are a few ways to do that, but the recommended way isthe "rootMode" option with "upward", which will make Babel search fromthe working directory upward looking for your babel.config.js file,and will use its location as the "root" value.

One helpful way to test if your config is being detected is to place a console.log() callinside of it. Since it is a JS file, the log will execute the first time Babel loads it.

How you set this value varies by project, but here are a few examples:

CLI

babel --root-mode upward src -d lib

@babel/register

require("@babel/register")({
  rootMode: "upward"
});

Webpack

module: {
  rules: [{
    loader: "babel-loader",
    options: {
      rootMode: "upward",
    }
  }]
}

Jest

Jest is often installed at the root of the monorepo and may not require configuration,but if it is installed per-package it can unfortunately be more complex to configure.

The main part is creating a custom jest transformer file that wraps babel-jest's defaultbehavior in order to set the option, e.g.

module.exports = require("babel-jest").createTransformer({
  rootMode: "upward",
});

and with that saved somewhere, you'd then use that file in the place of babel-jest inyour Jest options via the transform option:

"transform": {
  "^.+\\.jsx?$": "./path/to/wrapper.js"
},

so all JS files will be processed with your version of babel-jest with the option enabled.

Others

There are tons of tools, but at the core of it is that they need the rootMode option enabledif the working directory is not already the monorepo root.

Subpackage .babelrc files

Similar to the the way babel.config.js files are required to be in the "root",.babelrc files must be in the root package, by default. This means that, the same way theworking directory affects babel.config.js loading, it also affects .babelrc loading.

Assuming you've already gotten your babel.config.js file loaded properly as discussed above,Babel will only process .babelrc files inside that root package (and not subpackages),so given for instance

package.json
babel.config.js
packages/
  mod/
    package.json
    .babelrc
    index.js

compiling the packages/mod/index.js file will not load packages/mod/.babelrc becausethis .babelrc is within a sub-package, not the root package.

To enable processing of that .babelrc, you will want to use the "babelrcRoots"option from inside your babel.config.js file to do

babelrcRoots: [
  ".",
  "packages/*",
],

so that Babel will consider all packages/* packages as allowed to load .babelrc files,along with the original repo root.

Config Format

The format of individual config files themselves separates into JS files vs JSON5 files.

JSON5

Any file that isn't a .js file will be parsed as JSON5 and should contain an object matchingthe options format that Babel accepts.

JavaScript

Any .js file will be require()ed and should export either a configuration object, or a functionthat will return a configuration object when called. The main benefit being that users can includeJS logic to build up their config structures, potentially allowing config logic to be sharedmore easily. .js files can be used as project-wide configuration orvia .babelrc.js files for file-relative configuration.

Function-returning configs are given a few special powers because they can access an API exposedby Babel itself. See Config Function API for more information.

Config Function API

JS config files may export a function that will be passed config function API:

module.exports = function(api) {
  return {};
}

The api object exposes everything Babel itself exposes from its index module, along withconfig-file specific APIs:

api.version

Type: string

The version string for the Babel version that is loading the config file.

api.cache

JS configs are great because they can compute a config on the fly, but the downsidethere is that it makes caching harder. Babel wants to avoid re-executing theconfig function every time a file is compiled, because then it would also need tore-execute any plugin and preset functions referenced in that config.

To avoid this, Babel expects users of config functions to tell it how to managecaching within a config file.

• api.cache.forever() - Permacache the computed config and never call the function again.
• api.cache.never() - Do not cache this config, and re-execute the function every time.
• api.cache.using(() => process.env.NODE_ENV) - Cache based on the value of NODE_ENV.Any time the using callback returns a value other than the one that was expected, the overallconfig function will be called again and a new entry will be added to the cache.
• api.cache.invalidate(() => process.env.NODE_ENV) - Cache based on the value of NODE_ENV.Any time the using callback returns a value other than the one that was expected, the overallconfig function will be called again and all entries in the cache will be replaced with the result.
• api.cache(true) - Same as api.cache.forever()
• api.cache(false) - Same as api.cache.never()

Since the actual callback result is used to check if the cache entry is valid, it is recommendedthat:

• Callbacks should be small and side-effect free.
• Callbacks should return values with the smallest range possible. For example, the.using(() => process.env.NODE_ENV) usage above is not ideal because it would create an unknownnumber of cache entries depending on how many values of NODE_ENV are detected. It would besafer to do .using(() => process.env.NODE_ENV === "development") because then the cache entrycan only ever be true or false.

api.env(…)

Since NODE_ENV is a fairly common way to toggle behavior, Babel also includes an API functionmeant specifically for that. This API is used as a quick way to check the"envName" that Babel was loaded with, which takes NODE_ENV into accountif no other overriding environment is set.

It has a few different forms:

• api.env("production") returns true if envName === "production".
• api.env(["development", "test"]) returns true if ["development", "test"].includes(envName).
• api.env() returns the current envName string.
• api.env(envName => envName.startsWith("test-")) returns true if the env starts with "test-".

Note: This function internally makes use of api.cache mentioned above to ensure that Babel is aware that this build depends on a specific envName. You should not use it alongside with api.cache.forever() or api.cache.never().

api.caller(cb)

This API is used as a way to access the caller data that has been passed to Babel.Since many instances of Babel may be running in the same process with different callervalues, this API is designed to automatically configure api.cache, the same way api.env() does.

The caller value is available as the first parameter of the callback function. It is best usedwith something like

function isBabelRegister(caller) {
  return !!(caller && caller.name === "@babel/register");
}
module.exports = function(api) {
  const isRegister = api.caller(isBabelRegister);
  return {
    // ...
  };
}

to toggle configuration behavior based on a specific environment.

api.assertVersion(range)

While api.version can be useful in general, it's sometimes nice to just declare your version.This API exposes a simple way to do that with:

module.exports = function(api) {
  api.assertVersion("^7.2");
  return {
    // ...
  };
};