Dual Publish

Publish JS project as dual ES modules and CommonJS package to npm.

• Tested on projects with 16M downloads per month.
• CI tests with it works with Node.js, browsers, React Native,
  bundlers (webpack, Parcel, Rollup, or esbuild) and CDNs (like jspm).
• Does not change line numbers in stacktrace and keep sources readable.
• No build step. No need for separated src/ and dist/ dirs in repository.
  You will be able to test branch by installing version from GitHub like
  npm i example@you/example#fix.
• Multiple files support. Your user will be able to import separated files
  
  like import { nanoid } from 'nanoid/async'.
• Cleans npm package from development configs before publishing.
• Supports process.env.NODE_ENV for development checks, which you want
  to remove in production JS bundle.

You write CommonJS in your npm library sources:

// index.js
module.exports = { lib }

npx dual-publish compiles your library during publishing to npm:

// index.js
export { lib }

// index.cjs
module.exports = { lib }

// package.json
{
  …
  "type": "module",
  "module": "index.js",
  "main": "index.cjs",
  "exports": {
    "require": "./index.cjs",
    "import": "./index.js"
  }
}

Now your library can be imported natively as ESM or CommonJS:

// CommonJS
let { lib } = require('lib')

// ESM in Node.js, webpack, Parcel, and Rollup
import { lib } from 'lib'

// ESM in browser
import { lib } from 'https://cdn.jsdelivr.net/npm/lib/index.js'

Limits

• We recommend to avoid default export because of bug in webpack.
• Every JS file should be in own dir. lib/index.js instead of lib.js.
  We need it to put package.json with module.

Usage

1. Take a normal CommonJS project with require() and module.exports =.

2. Because of bug in webpack we recommend to use only named exports:

   const NAME = 'a'
   function change {
     …
   }
   
   module.exports = { NAME, change }
   

3. Move all files into separated dirs. Rename lib.js to lib/index.js.
   Old require('./lib') will work.

4. Add dual-publish to the project:

   npm i --save-dev dual-publish
   

5. Test the result by calling npx dual-publish --check.
   It will create a folder in your project with converted files.
   Review them manually.

6. Publish your project with npx dual-publish instead of npm publish.

   npx dual-publish