Types are CJS, but implementation is ESM

[neurosnap]

Greetings! This project is awesome, thanks for all the work that has gone into it!

I recently stumbled across a project that attempts to scan an npm package for typescript type issues. Since I'm using dnt to generate the npm package for me, I was wondering if you had any guidance on how to fix these issues:

• Results from: https://arethetypeswrong.github.io/?p=starfx%400.0.4
• dnt script for project: https://github.com/neurosnap/starfx/blob/main/npm.ts

I'm mostly concerned about node16 (Masquerading as CJS). It looks like we need to have specific esm types .d.mts in our package.exports.

From the github: https://github.com/arethetypeswrong/arethetypeswrong.github.io

Types are CJS, but implementation is ESM. In the node16 resolution mode, TypeScript detects whether Node itself will assume a file is a CommonJS or ECMAScript module. If the types resolved are detected to be CJS, but the JavaScript resolved is detected to be ESM, this problem is raised. It is often caused by a dependency’s package.json doing something like:

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

Because there is no "type": "module" setting here, the .js and .d.ts file will always be interpreted as a CommonJS module. But if the importing file is an ES module, the runtime will resolve to the .mjs file, which is unambiguously ESM. In this case, the module kind of the types misrepresents the runtime-resolved module. This tends to present the most issues for users when default exports are used. In the future, this tool may detect whether an export default might make this problem more severe and give a full explanation of why. In the meantime, you can read the explanation in microsoft/TypeScript#50058 (comment). The simple fix for the example above would be to add an index.d.mts file dedicated to typing the .mjs module, and remove the "types" condition.

Thanks again!

[dsherret]

Upgrade to https://github.com/denoland/dnt/releases/tag/0.36.0 or set declaration: "inline" -- that should fix the issue.

I'm not sure about the node 10 issues, but node < 16 no longer has LTS so people should be upgrading anyway https://nodejs.dev/en/about/releases/

[neurosnap]

Ahh! I should have looked at releases, thanks so much!

[dsherret]

No worries! It's good to have an issue for this.