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What's your use case?

There are two main options:

  • babel.config.json
  • .babelrc. The extension can also be .babelrc.js, .babelrc.cjs, or .babelrc.mjs.

babel.config.json

Example:

{
  "presets": [
    "@babel/preset-env"
  ],
  "plugins": [
    "@babel/plugin-transform-runtime",
    "@babel/plugin-proposal-class-properties"
  ]
}

In this example:

@babel/preset-env is used to apply a collection of plugins for modern JavaScript syntax. Additional plugins (@babel/plugin-transform-runtime and @babel/plugin-proposal-class-properties) are added to handle specific transformations not covered by the preset.

Presets and Plugins

Presets are collections of plugins bundled together. They are essentially a way to simplify configuration by grouping together a set of plugins that target a specific environment or set of features. Presets allow you to apply multiple plugins with a single configuration entry. If you need to transform specific parts of your code and have very precise requirements, you might choose to specify individual plugins.

targets

This field describes the environments you support/target for your project. This can either be

  • a browserslist-compatible query or
  • an object of minimum environment versions to support

Supported environments: android, chrome, deno, edge, electron, firefox, ie, ios, node, opera, rhino, safari, samsung.

If a minor version is not specified, Babel will interpret it as MAJOR.0.

For example,

{
  "targets": {
    "chrome": "58",
    "node": 12
  }
}

will be considered as Node.js 12.0.

When no targets are specified: Babel will assume you are targeting the oldest browsers possible. For example, @babel/preset-env will transform all ES2015-ES2020 code to be ES5 compatible.

JSON vs. JS formats

The Babel people recommend using the json file type versus the JS type:

  • JS config files are handy if you have complex configuration that is conditional or otherwise computed at build time. However,
  • The downside is that JS configs are less statically analyzable, and therefore have negative effects on cacheability, linting, IDE autocomplete, etc.
  • Since babel.config.json and .babelrc.json are static JSON files, it allows other tools that use Babel such as bundlers to cache the results of Babel safely, which can be a huge build performance win.

Options

The list of options is here: https://babeljs.io/docs/options

plugin entry

The entry plugin is an array of plugins to activate when processing this file.

Babel's configuration merging is relatively straightforward:

  • options will overwrite existing options when they are present.
  • 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.

See https://babeljs.io/docs/options#config-merging-options

preset entry

The presetentry is an array of presets to activate when processing.

Note: The format of presets is identical to plugins, except for the fact that name normalization expects "preset-" instead of "plugin-", and presets cannot be instances of Plugin.

Code Generation options

See https://babeljs.io/docs/options#code-generator-options and https://babeljs.io/docs/babel-generator#options

Working with Monorepos

Monorepo-structured repositories usually contain many packages. With monorepo setups, the core thing to understand is that Babel treats your working directory as its logical "root", which causes problems if you want to run Babel tools within a specific sub-package without having Babel apply to the repo as a whole.

JS Config Files

JS config files may export a function that will be passed config function API. See https://babeljs.io/docs/config-files#config-function-api:

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

References

The documentation: https://babeljs.io/docs/config-files