npm-packages.md

What will future hybrid npm packages look like?

Status quo

It’s great to see how many npm packages already come with ESM versions. Most common seems to be:

"main": "src/main.js",
"module": "dist/mypkg.esm.js",

import {x} from 'mypkg'; // bundler uses "module" entry
const {x} = require('mypkg');

Files in package:

mypkg/
  src/
    main.js  # CommonJS
    util.js  # CommonJS
  dist/
    mypkg.esm.js  # bundled

Future (once ESM is unflagged in Node.js)?

"main": "src/main.mjs",

import {x} from 'mypkg';
const {x} = require('mypkg/cjs');

Files in package:

mypkg/
  src/
    main.mjs
    util.mjs
  cjs/
    index.cjs  # main
    util.cjs

Edits

• Rewrite, based on suggestions by @mgtitimoli

I believe it would better to have a cjs folder under the package with a package.json with the "type": "commonjs" since all the files under this folder are gonna be of this type.

So at the end the user would be able to require it like: const mypkg = require("mypkg/cjs");

@mgtitimoli True!

I don't know if they are gonna be bundled, but transpiled for sure...

CJS output would require more transformations as it would be targeted for older engines.

Thanks @rauschma for including my suggestions, however there are still some things to clarify:

1. All the files would have .js extensions (<- this is super important) and their types are gonna be controlled by the closer package.json
2. The main package.json will have an entry "type": "module", so this way all files would be treated as modules by default
3. There gonna be a cjs folder with an additional package.json but this time this one will have an entry with "type": "commonjs", so every file under this folder would be treated as a commonjs file.

It's worth noticing that I came out with these suggestions after having read this some time ago.

Probably unpopular idea: what about publishing multiple packages, each with different language versions?

mypkg-cjs
mypkg-es2016

Build tools could help with automated publishing and/or NPM could support this somehow.

This would avoid unused JS files in node_modules and would not require any specialized support by Node nor bundlers.