Use native ESM for dev scripts

[karansapolia]

Q	A
Fixed Issues?	Fixes #12137
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?
Documentation PR Link
Any Dependency Changes?
License	MIT

[AjayPoshak]

@karansapolia My bad, I did not notice that this PR is still in draft.

[nicolo-ribaudo]

@karansapolia Do you need any help with this? 😄 If you don't have time/interest, I could finish your commits.

(I'm sorry to ping you, but this PR is super exciting for me)

[nicolo-ribaudo]

1. Since packages/babel-types/scripts only contain native modules, we can put a package.json file there like this one:

   {
     "type": "module"
   }

   and use the .js extension rather than .mjs.
2. Gulpfile.js doesn't need to be renamed to Gulpfile.mjs
3. babel.config.js must be renamed to babel.config.cjs
4. We can rename the files in the test/esm folder to .js, and update the path in the "test:esm" script of the top-level package.json

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/38935/

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[nicolo-ribaudo]

PnP doesn't support ESM yet, but they are working on it: yarnpkg/berry#2161

(:frowning_face: since we only enabled PnP for this CI Job yesterday)

I personally wouldn't expect them to introduce support for ESM by default until Node.js' loader API becomes stable (currently it's --experimental-loader), but:

• We could decide to use feat(pnp): experimental esm support yarnpkg/berry#2161 instead of their last release: Yarn is committed into our repo, so it will not "suddently break" anyway.
• They could release ESM support even if loaders are still experimental, maybe behind an experimentalESM config option (cc @merceyz, just in case you like this suggestion 😛)
• They already use our repository in their E2E tests for node_modules: also giving them a real-world repository that use Yarn 2 to test their ESM support might help with getting ESM support sooner and/or make it more stable!

[merceyz]

All that PR needs is someone to test it but none of the maintainers actually use ESM in our projects so we can't dogfood it, would be great if a project like Babel could test it 👍

[nicolo-ribaudo]

I'll try using it this week and report feedback!

[merceyz]

I've added the config enableExperimentalESMLoader to that PR so it can be turned on without setting the root package.jsons type field to module.

I'll try using it this week and report feedback!

Thanks, if you got any questions or anything I'm usually active on Discord 👍

[JLHwung]

Any updates on ESM support in Yarn 2? IMO the PnP check is important in case we forget declaring dependencies. We could add a new build job which transpiles Gulpfile.mjs and then build with PnP.

[nicolo-ribaudo]

Because we only lint src/ files with that rule 😬

[nicolo-ribaudo]

It's less important to lint other things since they will never affect our users, but we can lint everything if you prefer.

[JLHwung]

👍 to lint everything except git-ignored.

[merceyz]

Any updates on ESM support in Yarn 2?

@JLHwung It works as far as I can tell, @nicolo-ribaudo was testing it on the Babel repo but I don't know the result of that test. All the issues that were brought up was fixed as far as I can tell

[nicolo-ribaudo]

Yup, in this repo it's working (or at least, it was a few weeks ago).
However, I didn't want to put any pressure on yarn since the ESM loader API is far from being stable.

[nicolo-ribaudo]

I rebased and fixed the remaining build errors. Thanks @karansapolia for starting this PR! 🧡

[karansapolia]

@nicolo-ribaudo Thank you to the babel team for the opportunity. This has stretched a lot and needed a lot of reviews and work from you. Lot to learn from you all. Thank you.

[JLHwung]

LGTM as long as CI is green.

[nicolo-ribaudo]

I restarted the failing job, it was a network error.