Add compatibility with TypeScript and ESM

[jscheid]

This adds compatibility with TypeScript's Experimental Support for ECMAScript Modules in Node.js. I've seen the package.json syntax mentioned in microsoft/TypeScript#33079 and it solves the issue for me.

It's meant as an example only, you might want to hold off merging this until TypeScript 4.7 is released. I'm not sure this will continue to work in the release. And if it does, you might want to do something similar for all the other packages in this repository. I've marked it as draft for these reasons.

Also, I haven't checked whether the syntax in index.d.mts would work with other TypeScript versions and configurations. If it does then it would be better to use a single file instead of both index.d.ts and index.d.mts.

And finally, I haven't checked whether TypeScript 4.7 without ESM support enabled also needs the different syntax. If so, it might be necessary to use the typesVersion setting.

Fixes #428

[angus-c]

Thanks @jscheid. I will hold onto this pending 4.7 release and checking the results in various environments (and pending me taking the time to understand it fully 😄 )

[adam-coster]

I just ran into this today since I'm using NodeNext for my Typescript setup. If I understand it all correctly, the crux of the issue is that the existence of the exports field makes it so that only the exports field will be used (and main, types, etc end up ignored). So in the case of this library:

• There is an exports field, so the root-level types field is ignored by latest Typescript/Node versions
• There is no exports['.'].types field, so Typescript falls back to look for a sibling file to the imported one (e.g. for index.js it looks for index.d.ts, while for index.mjs it looks for index.d.mts).
• There is no sibling index.mjs file, so Typescript gives up trying to find types.

The current PR solves this by adding that index.d.mts file, and also adding that to the exports values, but there is a simpler solution: just add the current types filepath to the exports['.'].types field of the package.json:

{
  "exports": {
    ".": {
      // Just need to add the following line
      "types": "./index.d.ts",
      "require": "./index.js",
      "default": "./index.mjs"
    }
  }
}

In essence, that fixes the fact that the root-level types field gets ignored when the exports field is present.

[adam-coster]

Here's the diff I got when using patch-package to fix this locally:

diff --git a/node_modules/just-pick/package.json b/node_modules/just-pick/package.json
index 50223ce..256d586 100644
--- a/node_modules/just-pick/package.json
+++ b/node_modules/just-pick/package.json
@@ -7,7 +7,8 @@
   "exports": {
     ".": {
       "require": "./index.js",
-      "default": "./index.mjs"
+      "default": "./index.mjs",
+      "types": "./index.d.ts"
     }
   },
   "types": "index.d.ts",

[angus-c]

Hi @adam-coster. Sorry for the belated reply. This does sound like the best solution.
I will create a new PR that batches this change for all modules (unless someone else does it first 😉 )
Thanks @adam-coster and @jscheid for investigating!