Add mjs files for proper esm support

[siilike]

Currently in order to use the esm version of the package one needs to manually import/require the relevant thing from under esm/. This is inconvenient and doesn't work as expected in some situations.

Previously there was a pull request to add the "module" field to package.json to make tools aware where the esm modules are, but was turned down.

This pull request adds the relevant mjs files, so when the bundler/runtime is configured to prefer them over js files the esm version will be used.

[ljharb]

To properly support ESM requires the “exports” field, and adding that is always a breaking change.

Adding the files in the interim would be semver-minor, but then they’d require using extensions, and adding the exports field would break that (since the explicit exports entry paths would ofc require extensions be omitted)

[ljharb]

this would break every single consumer, including ESM ones, using the specifier react-dates

[siilike]

Why, when it defaults to index?

[ljharb]

ah, that's a fair point, i never rely on the default so i forgot that.

That said, explicit > implicit, so altho this part isn't a breaking change, let's please revert it.

[siilike]

Yes, but then it's not going to work, because it will always load the index.js file regardless of extension preference. Then it makes more sense to add the "exports" field instead.

Or perhaps revisit the idea of adding the "module" field? It's not standardized, but still widely used and very unlikely to break anything.

[ljharb]

I'm confused. The default is index.js, whether you import or require it.

[siilike]

The default is index.something, node supports index.js, index.json, index.node, perhaps also something else. Webpack can be configured to prefer mjs over others, so it would use index.mjs.

But now thinking about it, it doesn't really sound like a good idea any more.

[ljharb]

That's not accurate - in CJS, extension resolution applies, but not in ESM. This means that index.js (in a package without type: module) is always CJS, and import('react-dates') continues to work, with or without it, by importing the CJS file.

The only resolution a package should expect is node's resolution. Using "exports", we can provide an ESM file, and then tools like webpack can use those as they like.