module: fix extensionless entry with explicit type=commonjs

[inoway46]

When an extensionless entry point contains ESM syntax but is in a package with "type": "commonjs" in package.json, the module would silently exit with code 0 without executing or showing any error. This happened because extensionless files skip the .js suffix check in the CJS loader, so the explicit type: commonjs was not being enforced, allowing ESM syntax to be silently delegated to ESM loading which never completed before the process exited.

This change ensures the CJS loader treats extensionless entry points as commonjs when type is explicitly set to "commonjs" in package.json, forcing ESM syntax to surface as a SyntaxError instead of silently exiting.

Fixes: #61104

Related: #61171 (alternative approach)

[nodejs-github-bot]

Review requested:

• @nodejs/loaders

[joyeecheung]

Thanks for the PR, a couple of comments, otherwise this looks good.

[joyeecheung]

Hmm, why is --no-experimental-detect-module needed here? I think the original issue is that it fails by default (without this flag)?

[joyeecheung]

As far as I can tell the test should be like this:

spawnSyncAndAssert(process.execPath, [fixtures.path('es-modules', 'extensionless-esm-commonjs', 'script'), {
  status: 1,
  stderr: /SyntaxError: Cannot use import statement outside a module/
})

[inoway46]

Thanks for the review.

I initially thought --no-experimental-detect-module was needed because I accidentally ran the test with a stale local build from another branch. That build did not include the extensionless entry-point handling change, so the extensionless script under type: "commonjs" was still being treated via ESM syntax detection.

After rebuilding on the correct branch (with the fix applied), I confirmed the flag is not necessary. The extensionless script is handled as CommonJS and the expected syntax error is produced. I also updated the test to the inline style you suggested and verified it passes.

Appreciate the guidance.