fix: export module with .mjs to fix Vite resolution

[kevinji]

Vite tries to resolve the module as a CommonJS module because the main package.json does not contain "type": "module". Override this behavior by explicitly adding a .mjs extension.

Locally, this fixes vega/svelte-vega#977 for me.

[domoritz]

Thanks for the pull request. Isn't this incorrect behavior from vite? I thought module and import imply esm.

[kevinji]

I'm unsure what the correct behavior based on the spec should be, but based on this Node.js package example, it claims:

The preceding example uses explicit extensions .mjs and .cjs. If your files use the .js extension, "type": "module" will cause such files to be treated as ES modules, just as "type": "commonjs" would cause them to be treated as CommonJS. See Enabling ESM.

[domoritz]

This hasn't come up in years and we have multiple packages that I want to keep consistent so I'm treading carefully here.

Maybe the best move at this point is to cut new major versions and move on to esm for everything.

[kevinji]

I understand your concern about not special-casing this repo. My guess is that this was more recently broken by #1301, as vega and vega-lite use the older module field without exports and this seems to be handled correctly.

This also seems consistent with the documentation under Conditional exports, which says

"import" - matches when the package is loaded via import or import(), or via any top-level import or resolve operation by the ECMAScript module loader. Applies regardless of the module format of the target file. Always mutually exclusive with "require". [emphasis added]

which suggests that the target file would default to CommonJS if "type": "module" is missing, even though import is used to load the package.

For consistency, it may be easier to move all repos to "type": "module" and explicitly output .cjs files for CommonJS compatibility; I have no strong preferences other than any fix for ESM seems better than none.

[kevinji]

@domoritz did you have a preference for how you wanted to move forward? I can make a larger PR to convert all the Vega projects to using "type": "module" if you prefer, although those will be a bit more invasive as all the other .js files will need to be converted to ESM.

[domoritz]

Yeah, I think moving to ESM for everything is a good idea: vega/vega#3990.

We already use import statements so hopefully it's a not too painful transition. I'm a bit busy until the end of the semester but plan to make some time over the break to get this done. It would be awesome if you could help with PRs!