[Experimental npm support] npm package implicit entry point not resolved correctly (cross-fetch)

[karelklima]

Popular cross-fetch polyfill NPM package (https://github.com/lquixada/cross-fetch) does not work with with the new npm: specifiers.

Example of code that results in an error

// Import from a package that has cross-fetch as a dependency
import { FetchDocumentLoader } from "npm:jsonld-context-parser";
const loader = new FetchDocumentLoader();

Error message

error: Uncaught Error: Cannot find module 'C:\...\deno\npm\registry.npmjs.org\cross-fetch\3.1.5\polyfill\dist\node-ponyfill.js'. Please verify that the package.json has a valid "main" entry
    at tryPackage (deno:ext/node/02_require.js:127:21)

Cause of the error

The cross-fetch polyfill is supposed to be imported as import 'cross-fetch/polyfill'. However, the package does not provide exports field in the root package.json. Instead there is a subfolder named polyfill containing a separate package.json file (https://github.com/lquixada/cross-fetch/blob/main/polyfill/package.json).

When resolving the module, Deno uses an entrypoint from the root package.json file, but also keeps the path to the subfolder in the module name, resulting in final path cross-fetch/3.1.5/polyfill/dist/node-ponyfill.js instead of cross-fetch/3.1.5/dist/node-ponyfill.js.

Possible solutions

1. Let Deno resolve package.json files from subfolders in npm modules.
2. Upstream fix - adding correct exports entry to cross-fetch fixes the issue, but it is probably not happening. There is an open PR from mid-2021: Support native node esm imports lquixada/cross-fetch#111

The cross-fetch package is quite a big deal as it has 9.6 million weekly downloads on NPM (react has 15.7 million).

Temporary solution

I have managed to make the package work by modifying its local copy in Deno cache by applying changes contained in the aforementioned PR: https://github.com/lquixada/cross-fetch/pull/111/files

Another possibility would be to remap the module using an import map to e.g. an empty module, but it is probably not yet supported with the new npm: specifiers (I wasn't able to make it work).

[bartlomieju]

@karelklima I just tried the reproduction you provided and I don't get any error thrown (Deno v1.29.1). I believe this has been already fixed. I'm gonna close the issue for now, let me know if you encounter any more problems.

[karelklima]

@bartlomieju Thank you for fixing this! I will resume working on the project where I encountered the problem and will let you know if there are any other similar issues.