Consider supporting an array for package.json types

Search Terms

package.json types

Suggestion

Allow the package.json "types" field to be a list of strings.

Use Cases

I will acknowledge that the primary use case inspiring this suggestion is a bit flimsy (which I will address below), but allowing the "types" field to be a list can potentially solve a current issue with monorepo development, by giving the compiler fallbacks for either the development / tooling stage or the build stage.

Examples

Take a monorepo project structure:

/repo/tsconfig.base.json
/repo/package.json

/repo/packages/App/package.json
/repo/packages/App/src/tsconfig.json
/repo/packages/App/src/index.ts

/repo/packages/LibA/package.json
/repo/packages/LibA/src/tsconfig.json
/repo/packages/LibA/src/index.ts

/repo/packages/LibB/package.json
/repo/packages/LibB/src/tsconfig.json
/repo/packages/LibB/src/index.ts

// /repo/tsconfig.base.json
{
  "compilerOptions": {
    "baseUrl": "./",
    "module": "esnext",
    "moduleResolution": "node",
    "paths": {
      "@repo/*": ["node_modules/@repo/*/src"]
    }
  }
}

For each packages src/tsconfig.json:

{
  "extends": "../../../tsconfig.base.json"
}

"App" isn't really an issue because nothing imports it. But the libs could be used in multiple ways, and can also be published. For publishing, a lib might have a package.json file like:

{
  "name": "@repo/LibA",
  "version": "0.0.1",
  "main": "index.js",
  // This is built
  "types": "index.d.ts"
}

With this configuration, builds work fine. However, during development, we don't want to reference a compiled definition file for types. If we make changes to a lib, we want to see those changes without having to compile and refresh any caches or a TypeScript server.

To accomplish this, we can just use a clean script to clear out any of the build artifacts during development, but there's a slight problem: Without the definition file (e.g. /repo/packages/LibA/index.d.ts), imports aren't exactly what we're looking for by default. For example, without the definition file, if I try to auto import, either in VSCode or WebStorm, it looks like this:

// /repo/packages/App/src/index.ts
import {Foo} from '@repo/LibA/src';

let f: Foo // Auto import triggered

So now I have to go trim off the /src b/c TS is looking for types, and isn't finding them. I'm actually not sure why it adds the /src, even though we have path mappings that would resolve to a shorter path. What I do know is that we can fix this during development by changing the "types" field:

{
  "types": "src/index.ts"
}

According to descriptions of the module resolution algorithm and looking through the code that does this resolution, even for DtsOnly, TypeScript files are fine. Now auto imports will default to the expected behavior:

import {Foo} from '@repo/LibA';

let f: Foo

I think now it's probably obvious why I think a "types" array would help solve this. If types were a list, we could clean build files during development and still fallback to sources for types.

There are a lot of potential issues with this, obviously. I think the most I can probably hope for is to draw attention to yet another issue with TypeScript and monorepos. I've read through most of this long discussion, and monorepo woes are obviously something the team is aware of.

Some issues I can think of:

• It might encourage authors to export types in ways that aren't encouraged by the TS team or the community.
• If multiple type files are present, it could incur a significant performance penalty from excessive parsing and declaration merging
• It's a non-targeted solution for a very targeted problem that few people probably contend with anyways.

However, by bringing it up, I figure that the experts here will understand the implications and potential solutions on a much deeper and more intuitive level. Perhaps there is some way of supporting this without adding anything gnarly or changing the "types" field. Potentially a "typesFallback" field could be added. I don't know what's worse, honestly.

Thank you for your time.

Checklist

My suggestion meets these guidelines:

• [✓] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [✓] This wouldn't change the runtime behavior of existing JavaScript code
• [✓] This could be implemented without emitting different JS based on the types of the expressions
• [✓] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

No implementation files or tests are named in the issue. Start by tracing TypeScript's package.json type resolution and review the linked discussion in issue #25376. The work would be complete when the supported package.json representation and fallback behavior are specified, implemented, and covered by resolution tests.