Flag to type `Promise.catch` variables as `unknown`

[yoav-lavi]

Suggestion

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Following #41016 a flag was added to consider catch variables as unknown.
For consistency and since they're similar cases, I suggest also adding a flag that does this for .catch callbacks on promises.

See also: eslint-plugin-etc/no-implicit-any-catch

[andrewbranch]

The type for .catch(...) callback parameters is defined in lib files, so we’d have to expose a new type that acts like unknown or any depending on compilation settings... and I’m not sure we’d want that type escaping into the wild 😬. This is a compelling use case, though.

[Cellule]

You can easily achieve this by overloading the signature of Promise.catch in your project

// Promise.d.ts
interface Promise<T> {
  /**
   * Attaches a callback for only the rejection of the Promise.
   * @param onrejected The callback to execute when the Promise is rejected.
   * @returns A Promise for the completion of the callback.
   */
  catch<TResult = never>(
    onrejected?: ((reason: unknown) => TResult | PromiseLike<TResult>) | undefined | null,
  ): Promise<T | TResult>;
}

[yoav-lavi]

@Cellule Thanks for the snippet, yes, you can do this yourself, but the goal of this suggestion is to add this to TS itself

[fregante]

@Cellule thanks for that! Unfortunately it doesn't cover allSettled though and I wasn't able to override that:

interface PromiseRejectedResult {
  status: "rejected";
  // @ts-expect-error TS2717 even if overridden, it doesn't work
  reason: unknown;
}

https://www.typescriptlang.org/play?target=7#code/IYZwngdgxgBAZgV2gFwJYHsIwG4FMBOYehEqUuAFAJQwDeAUDEzFJiMjAA77oC2qIXCBgBeGMADuwVBwAKPfoIB0wADaqAyrmTJVuACYUA2gEYAulUbM46fDAqsI7LgoG4Y6OC75uQNBszMqF4U3D7K7MDICMIicTAARPi4AFa4UMgGCf5WgUxhirhKyaCYucwAvuVVFUA

[emartinfigma]

You can easily achieve this by overloading the signature of Promise.catch in your project

I tried this, but it seems like TS just selects the 'any' overload if the signatures don't match. Playground

[thw0rted]

I'd really like to see this implemented, and I think it's a shame it wasn't considered for release alongside useUnknownInCatchVariables. Before the flag existed, you could always freely convert between

• p.then(good, bad)
• p.then(good).catch(bad), and
• try { result = await p; good(result) } catch (err) { bad(err); } (in async context)

Now, if bad doesn't expect unknown it will fail, but only in the last of these forms. I want a flag that says "use unknown type for all errors", including try/catch and Promise rejection -- which are the same thing, under async/await!

[ExE-Boss]

@thw0rted

• p.then(good, bad)
• p.then(good).catch(bad)

Are not the same, since if good throws an error, only the latter form will catch it.

---

p => p.then(good, bad) is actually equivalent to:

async p => {
	let result;
	try {
		result = await p;
	} catch (err) {
		return bad(err);
	}

	return good(result);
}

[thw0rted]

That's correct, but I didn't mean that those forms were identical, I was only referring to the parameter type of the bad function. The point is that the then-callback gets any while the catch-block invocation gets unknown.