[Bug?]: ERR_REQUIRE_ESM in Yarn PnP mode using Node.js 22 with *.mts file and --experimental-strip-types flag

[blutorange]

Self-service

• I'd be willing to implement a fix

Describe the bug

Trying to run a *.mt file via yarn node with no type or type: "commonjs" in the package.json fails and results in an ERR_REQUIRE_ESM error.:

node:internal/modules/cjs/loader:1314
      throw new ERR_REQUIRE_ESM(filename, true);
            ^

Error [ERR_REQUIRE_ESM]: require() of ES Module /home/user/git/project/a.mts not supported.
    at require$$0.Module._load (/home/user/git/project/.pnp.cjs:6014:31)
    at TracingChannel.traceSync (node:diagnostics_channel:315:14) {
  code: 'ERR_REQUIRE_ESM'
}

Judging from the error message require() of ES Module and .pnp.cjs. I'm guessing yarn currently just doesn't implement the logic for treating *.mts files as ESM and uses its .pnp.cjs instead of .pnp.loader.mjs ?

To reproduce

Make sure the package.json either has no type or "type": "commonjs".

Create two files:

// a.mts
import Message from "./b.mts";
console.log(Message);

// b.mts
const Hello = "World";
export default Hello;

Then run yarn node --experimental-strip-types a.mts. This will fail with an error ERR_REQUIRE_ESM'

Running node --experimental-strip-types a.mts works and prints World.

Environment

System:
    OS: Linux 6.8 Ubuntu 24.04.1 LTS 24.04.1 LTS (Noble Numbat)
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
  Binaries:
    Node: 22.11.0 - /tmp/xfs-ca62210f/node
    Yarn: 4.5.1 - /tmp/xfs-ca62210f/yarn
    npm: 10.9.0 - ~/.nvm/versions/node/v22.11.0/bin/npm

Additional context

Perhaps related to #6336 ?

[RDIL]

Hey @blutorange, can you please see if running yarn set version from sources --branch "reece/fix-mts" and then re-testing it resolves the issue?

[blutorange]

@RDIL It didn't, but I saw your commit c38fd4e and noticed that the change wasn't applied to .pnp.loader.mjs, even after I deleted that file and had yarn recreate it. I manually added your change regarding getFileFormat to my local copy of .pnp.loader.mjs. Then it worked : ) Probably just an issue with installing from sources.

So yes, that commit should be enough to fix it. It's probably best to also add .cts for commonjs

[RDIL]

Whoops, I forgot to rebuild the loader, my bad. Will also add CTS!

[goloveychuk]

@RDIL adding

case `.mjs`: {
      return `module`;
    }

to pnp loader fixes the issue, can you merge your PR? Thanks

[goloveychuk]

BUT, interestingly, strip-types argument does not work.

this "fixes.

Seems like stripping types is also done in loader, so you need to chain it.

[adrian-gierakowski]

@goloveychuk which file should your fix be applied to? Thanks!

[blutorange]

@adrian-gierakowski Just doing a search on this repo it's probably this file

berry/packages/yarnpkg-pnp/sources/esm-loader/hooks/load.ts

Lines 64 to 68 in 93a5664

	return {
	format,
	source: format === `commonjs` ? undefined : await fs.promises.readFile(filePath, `utf8`),
	shortCircuit: true,
	};

[goloveychuk]

@adrian-gierakowski
I've changed .pnp.loader.mjs, but to fix the issue for all, it should be what @blutorange mentioned