[feature] Generate esm-specific declaration files

[IanVS]

We are using tsup in storybook to bundle our files and create .d.ts files. A user, @weswigham, explained to us that we need to actually create separate type definition files for esm and cjs. Is this something that others have encountered? If so, maybe it makes sense for tsup to generate both .d.ts and .d.mts files.

It looks like this is a typescript 4.7+ feature, and is explained in https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#package-json-exports-imports-and-self-referencing, which shows separate declaration files for the esm and cjs exports.

The .d.mts and .d.cts extensions themselves are supported from 4.5+ (https://devblogs.microsoft.com/typescript/announcing-typescript-4-5-beta/#new-file-extensions)

[cyco130]

My current workaround is to use the following in the config file (I have the inverse issue):

async onSuccess() {
  await cp(
    resolve(__dirname, "dist/index.d.ts"),
    resolve(__dirname, "dist/index.d.cts"),
  );
},

(Needs more work for more complex libs, of course).

[RichiCoder1]

For some extra context: pmndrs/zustand#1381

TL;DR: TypeScript treats the below as CJS even when technically MJS is used under the covers when "type" is unspecified:

"export": {
  ".": {
    "types": "dist/index.d.ts",
    "import": "dist/index.mjs",
    "require": "dist/index.js"
  }
}

Or vice verse if the module is "type": "module".

So the missing part of tsup is to emit two different declarations in the above case (["dist/index.d.ts", "dist/index.d.mts"]) compiled with the correct target. Furthermore, the output of the Module (ESM) js and dts (when splitting) needs to have file extensions in any imports otherwise TypeScript will break when running under "target": "node16" or target "target": "nodenext".

[brc-dd]

The onSuccess thing doesn't work because of #700 👀

[cyco130]

The onSuccess thing doesn't work because of #700 eyes

Ooh, I must be copying the old d.ts then! Thanks for the heads up.

[Nishchit14]

any solution for this? It's breaking the type import in pnpm workspace with esm package.

[brc-dd]

Should be fixed by #934?