Type "unknown" for the reason in Promise.catch callback in lib.es5.d.ts

[etonee123x]

⚙ Compilation target

esnext

⚙ Library

lib.es5

Missing / Incorrect Definition

Promise.catch

Important:

strict mode is enabled. "useUnknownInCatchVariables" is not disabled.

Description

Check the sample code. It represents some fetch-request (0) and transformation of its results (1). For some reasons (inattention) I return error in Promise.catch (3).

What do I expect?

I expect that error (4.0) type (and then data (4.1) type) will be of type "unknown" and IDE will underline spreading data at transformation (1), typechecks will fail, etc --> I will notice the error --> I will prevent the error in runtime.

What do I get?

Error (4.0) type (and then data (4.1) type) are "any" and IDE didn't underline the error, typechecks didn't fail, etc --> I didn't notice the error --> I got TypeError in runtime, cos the error value (5) is not iterable.

Investigation

In other issue, where author was requesting for generic for rejected promise values, rbuckton agreed that type "any" is the most accurate type for rejected promises values. It was ~2-3 years before type "unknown" was introduced, though 🤔!..

Potential solution

Of course, I can add type annotation "error: unknown" in catch callback on my own, but I think it should be automatically inferred with flags I mentioned before. Or maybe create a new flag for this case. Or maybe change Promise.catch definition. Or maybe I don't understand something, please explain me!

Sample Code

// (0)
const getData = () => new Promise<Array<number>>(
  (resolve, reject) => Math.random()
    ? resolve([1, 2, 3])
    : reject(new Error('error!')), // (5)
);

(async () => {
  const data = await getData() // (4.1)
    .catch((error) => { // (4.0)
      console.log(error);
      return error; // (3)
    });

  return [0, ...data]; // (1)
})();

Documentation Link

No response

[MartinJohns]

Duplicate of #45602. Used search terms: promise catch in:title

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.