babel-dual-package

CLI for building a dual ESM and CJS package with Babel. Takes an ES module and produces a compiled ESM and CJS build using the configuration from your babel.config.json file.

Requirements

• Node >= 16.19.0.
• Your package uses "type": "module" in package.json.

Getting Started

First install babel-dual-package:

npm install babel-dual-package

Next write your babel.config.json file and include any plugins or presets your project requires.

{
  "presets": [
    ["@babel/preset-env", {
      "modules": false
    }]
  ]
}

Now run babel-dual-package src to get an ESM and CJS build in a dist directory that can be used as exports in a package.json file.

Run babel-dual-package --help to see a list of more options.

Examples

TypeScript and JSX

If your project is using typescript then add @babel/preset-typescript. If it is also using JSX, then add @babel/preset-react.

babel.config.json

{
  "presets": [
    ["@babel/preset-env", {
      "modules": false
    }],
    "@babel/preset-typescript"
    ["@babel/preset-react", {
      "runtime": "automatic"
    }]
  ]
}

Set the declarationDir for your types to the same value used for --out-dir, or dist (the default) if not using --out-dir.

tsconfig.json

{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "dist",
    "emitDeclarationOnly": true,
    "isolatedModules": true,
    "strict": true
  },
  "include": ["src"]
}

In order to support typescript, you must pass the --extensions used:

babel-dual-package --out-dir dist --extensions .ts,.tsx src

If everything worked you should get an ESM build in dist and a CJS build in dist/cjs with extensions in filenames and specifiers updated correctly.

Now you can add some scripts to your package.json file to help automate the build during CI/CD.

package.json

  "type": "module",
  "scripts": {
    "build:types": "tsc --emitDeclarationOnly",
    "build:dual": "babel-dual-package --out-dir dist --extensions .ts,.tsx src",
    "build": "npm run build:types && npm run build:dual"
  }

Flat build

Given a directory structure like the following,

app/
  package.json
  src/
    one/
      file1.js
    two/
      file2.js
    file.js

and by using --out-file-extension to make sure each build has unique filenames, you can create a flat build while still supporting conditional exports.

For example, running

babel-dual-package --no-cjs-dir --out-file-extension esm:.esm.js,cjs:.cjs.js src/*.js src/one src/two

Will produce the following build output:

dist/
  file.esm.js
  file.cjs.js
  file1.esm.js,
  file1.cjs.js,
  file2.esm.js,
  file2.cjs.js

Import query parameters

You can run a build by importing this package and passing an args query parameter in the import specifier. Boolean options like --no-cjs-dir should have an empty string as their value (''), and instead of positional arguments, you pass a files array (globs supported).

build.js

await import(
  `./node_modules/.bin/babel-dual-package?args=${encodeURIComponent(
    JSON.stringify({
      '--out-dir': 'dist',
      '--extensions': '.ts',
      '--no-cjs-dir': '',
      files: ['src/*.js']
    })
  )}`
)

Options

There are options that can be passed to provide custom output locations, file extensions, and more.

You can run babel-dual-package --help to get more info. Below is the output of that:

user@comp ~ $ ./node_modules/.bin/babel-dual-package --help
Usage: babel-dual-package [options] <files ...>

Options:
--out-dir [out] 		 Compile the modules in <files ...> into an output directory.
--root-mode [mode] 		 The project-root resolution mode. One of 'root' (the default), 'upward', or 'upward-optional'.
--cjs-dir-name [string] 	 The name of the --out-dir subdirectory to output the CJS build. [cjs]
--extensions [extensions] 	 List of extensions to compile when a directory is part of the <files ...> input. [.js,.jsx,.mjs,.cjs]
--out-file-extension [extmap] 	 Use a specific extension for esm/cjs files. [esm:.js,cjs:.cjs]
--keep-file-extension 		 Preserve the file extensions of the input files.
--no-cjs-dir 			 Do not create a subdirectory for the CJS build in --out-dir.
--no-comments 			 Remove comments from generated code.
--source-maps 			 Generate an external source map.
--minified  			 Save as many bytes when printing (false by default).
--copy-files 			 When compiling a directory copy over non-compilable files.
--help 				 Output usage information (this information).

Questions

Q Can I use "type": "commonjs" in my package.json file, or just not include the field?

A No. Converting from CJS to ESM is a codemod, not transpiling via Babel.