Non-JS extensions not always throwing in ESM resolver

[guybedford]

In #30514 (comment) @GeoffreyBooth found a scenario in which unsupported file extensions are still being resolved by the ESM resolver as modules.

We need to ensure that all unknown file extensions throw for the ESM resolver, as is the intention in the spec described in the ESM_FORMAT specification - https://nodejs.org/dist/latest-v13.x/docs/api/esm.html#esm_resolution_algorithm.

[adarshsaraogi]

hi I would like to work on this.

[guybedford]

I managed to track this one down and it is actually not a bug after all. @GeoffreyBooth when you called resolve(specifier) in the loader you only provided one argument to the resolver (whereas the resolver takes two arguments - the specifier, and the parent specifier. When no parent specifier is provided this is taken to mean isMain (Node.js main entry). Which results in a different resolution rule that extensions don't matter. This was done to support extensionless bin files and is correct by the specification isMain check in ESM_FORMAT. If you simply provide the parentUrl to the resolve call in your loader example there you will find it throws an error.

Thanks so much @adarshsaraogi for offering to help here, if you are interested in contributing there are some other issues labelled ES modules / help wanted / good first issue that I'd be glad to help you with.