Publishing an ES module for a library (extension .mjs?)

[cdauth]

I have recently migrated both my frontend and my Node.js backend to native ES modules. As part of this migration, I have run into trouble with several libraries that do not publish ES modules or that do not publish them properly, and I am in the process of opening pull requests and adapting my own libraries to improve this.

I have come across the Library section in the Vite docs. With the lib option, Vite provides a way to create both an UMD and a ESM bundle. The documentation recommends to publish the package with the following package.json:

{
  "name": "my-lib",
  "files": ["dist"],
  "main": "./dist/my-lib.umd.js",
  "module": "./dist/my-lib.es.js",
  "exports": {
    ".": {
      "import": "./dist/my-lib.es.js",
      "require": "./dist/my-lib.umd.js"
    }
  }
}

This looks exactly like the problem that I'm seeing in various libraries. Node.js interprets all .js files as CommonJS (unless "type": "module" is set in package.json), so the ES bundle is interpreted as CommonJS, causing an error that the export keyword is not defined. I believe that the ES bundle should have the .mjs extension. Even better, the UMD bundle should have the .cjs extension, to make sure that it also works in modules that have "type": "module" set, but I could imagine that that can cause problems for frontend libraries.

I think that this problem can easily be solved by changing the fileName function to ``fileName: (format) => `my-lib.${format}.${format === 'es' ? 'mjs' : 'js'}```. I am wondering whether I am misunderstanding something, or whether I should open an issue to correct the documentation, or whether this has already been discussed anywhere?

[grantholle]

I've just ran into this issue as a consumer of a package. It's set up correctly (according to the documentation) and very similar to yours. However, it doesn't work until the ESM file has an .mjs extension.

[cdauth]

I opened a pull request to correct the docs (#7714), it was rejected as this will be fixed in Vite v3 by #6827.