Typescript: `undefined` return type requires explicit return

[saschanaz]

TypeScript Version: playground

Search Terms: undefined return type

Code

function test(): undefined {
  console.log("hey")
}

Expected behavior: This should be allowed.

Actual behavior: TS requires the return type to be void or any.

Playground Link: https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABFApgZygCgJQC5HgAmKwMYKhiA3gFCKIQJpwA2KAdC3AOaYBEACxQBPPthoBfIA

[sirmmo]

void return type is not undefined. ok, javascript is so useless and badly thought that returning nothing and returning undefined are the same, but they are not at least not in typescript. undefined === void returns a syntax error. And that is because undefined is anyway something and not the absolute nothingness that is represented by void.

[annevk]

I think this might be a duplicate of #36288.

Given the behavior of

function test(): void {
  return undefined;
}

I think @saschanaz has a point, FWIW.

[saschanaz]

undefined === void returns a syntax error. And that is because undefined is anyway something and not the absolute nothingness that is represented by void.

AFAIK that's not because undefined is an actual value, it's because void is null | undefined (without --strictNullCheck) and thus it's inappropriate to blindly assign void to undefined.

[saschanaz]

I agree that this is a duplicate, closing.