It might be nicer to allow the config itself specify an output extension - then we could run twice, one with each config, to output for each extension (each of which has a different set of target environments)

@ljharb I think so too so that you could use a single cli command and change the output extension based on some environment variable.

I think this is a better starting point though since --keep-file-extension is also a cli flag.

Small nit in the PR description:

Webpack 4 automatically prioritizes foo.mjs over foo.js when resolving import foo from './bar'

Shouldn't the filenames be bar.mjs and bar.js (because of the path ./bar in the import example)?

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/11285/

Should I add a warning if both --keep-file-extension and --use-file-extension are used?

Yeah, it should throw.

WDYT about naming it --out-file-extension, for consistency with --out-file and --out-dir?

@nicolo-ribaudo

• refactored file extension logic (looked very brittle to me, path module already has everything we need: 680cab6
• renamed use-file-extension -> --out-file-extension: 003d658
• added error if both flags are used (do test these?): e12d5e5

You can test them like https://github.com/babel/babel/tree/master/packages/babel-cli/test/fixtures/babel/error%20--out-dir%20--out-file

@nicolo-ribaudo Done d5dc562. Initially forgot to add a trailing newline to stderr.txt and options.json. Added them later and squashed but github doesn't pick it up.

I would love to see this PR merged since it will allow us to remove ugly shell scripts from our package.json:
https://github.com/graphql/graphql-js/blob/0b5e95556ef8334416a1dcb97084c1cc0a21ff5e/package.json#L41-L42
and make our project more Windows-friendly.

ATM we have a choice of either waiting until this feature gets merged or drop babel-cli and write our build scripts using babel-core.
Can someone from Babel team please clarify the status of this PR?

Is there something blocking a merge?

Really looking forward to this shipping; so we can output separate .mjs and .cjs files with package scripts like this:

{
  "prepare:mjs": "BABEL_ESM=1 babel src -d . --out-file-extension .mjs",
  "prepare:cjs": "babel src -d . --out-file-extension .cjs"
}