gen-esm-wrapper

Generate ESM wrapper files for CommonJS modules

This CLI tool makes it easier for module authors to support both ES modules and CommonJS modules for Node.js. It looks at the exports of an existing CommonJS module, and creates an ES module wrapper file that makes the exports available as named exports.

For example, if you are in the root of your npm package, you can use

gen-esm-wrapper . path/to/wrapper/output.mjs

to create a wrapper file, and add

  ...
  "exports": {
    ".": {
      "require": "./<same file as 'main'>",
      "import": "./path/to/wrapper/output.mjs"
    },
    "./": "./"
  }
  ...

to your package.json in order to make your module available as a dual ESM/CJS package. The command can also be added as a build step before publishing.

The above example "exports": key can be simplified to

  ...
  "exports": {
    "require": "./<same file as 'main'>",
    "import": "./path/to/wrapper/output.mjs"
  }
  ...

if there are no other JavaScript files that can be required from the package besides the main entry point (e.g. a file named foo.js that can be loaded through require('package/foo')), or if you are okay with breaking require() calls to those modules. This is the recommended approach for new modules that are being published for the first time.

Example project

There is a minimal example in the example-project directory in this repository. This is its package.json:

{
  "name": "example-project",
  "version": "1.0.0",
  "description": "Example project for gen-esm-wrapper",
  "main": "index.js",
  "exports": {
    ".": {
      "require": "./index.js",
      "import": "./dist/output.mjs"
    },
    "./": "./"
  },
  "scripts": {
    "test": "npm run build && node ./dist/output.mjs",
    "build": "gen-esm-wrapper . ./dist/output.mjs",
    "prepack": "npm run build"
  },
  "devDependencies": {
    "gen-esm-wrapper": "^1.0.4"
  }
}