How to choose 'module' instead of 'main' file in package.json

Question

I have created some npm modules and compile them to:

• commonJS (using exports.default =) and
• esm (using export default)

I set up my package.json like so:

main: "index.cjs.js",
module: "index.esm.js"

When I npm install the package and I simple import it like:

import myPackage from 'my-package'

It will automatically choose the main file, not the module.

My question:

Is there a way to import the module file instead when doing import myPackage from 'my-package' in a JavaScript file?

Why I choose the commonJS file for "main":

I noticed that using Node, importing an esm file is not possible because of export default, it has to be commonJS. I have some simple helper JS functions like this and this, and I would want them to be usable to the widest audience. That's why I chose cjs for the "main" path in package.json.

Why I define a separate "module" in package.json:

Lots of famous libraries like Vue.js are already doing this. See further information on this Stackoverflow thread:

What is the "module" package.json field for?

Why do I want to be able to import the "module" file instead of the "main" file:

There is currently a bug in Rollup where it will not properly show JSDoc comments when coding after having imported a cjs file, but it does work when importing a es file.

The workaround I want to avoid:

Just set "main" to the esm file in package.json, right? But then all users who are using my packages in Node apps will not be able to use it anymore...

→　I'm really confused about all this as well, but I think I did enough research to make sense of all it. That being said, if anyone knows a better approach or any other advice, please do tell me in the comments down below!!

Answer 1

Just don't use extension for main file and have es6 and CommonJS version as two separate files with the same name and in the same directory, but with different extension, so:

index.js // transpiled CommonJS code for old nodejs
index.mjs // es6 module syntax

and in package.json:

{
  "main": "index"
}

If node is launched with --experimental-modules flag, it would use *.mjs file, otherwise *.js.

Answer 2

Nodejs does not support "module" but does support the newer "exports" spec.

https://nodejs.org/api/packages.html#exports

https://github.com/nodejs/node/blob/v16.14.0/lib/internal/modules/esm/resolve.js#L910

  "exports": {
    "import": "./main-module.js",
    "require": "./main-require.cjs"
  },