Simple conditional type as return type doesn't seem to work

[lament snow]

typescript cannot typecheck conditional return types

you will need to use as

[fleet prism]

you can try no matter what I did all fail

[lament snow]

as any

return (Math.random()>0.5 ? 1:2) as any;

[fleet prism]

as any yeah that might work. But as you say it seems TS has problems with conditional return types

Even this fails

[lament snow]

oh yeah that's a bit weird

i know that in some cases typescript widens it to IfTrue | IfFalse

so i wonder why it can't do IfTrue & IfFalse when going the other way

[naive ruin]

const index = <PK extends string, SK extends string | undefined, R = SK extends undefined ? 1 : 2>(pk: PK, sk?: SK): R => {
     return (Math.random() > 0.5 ? 1 : 2) as R;
}

this kinda works? but with const x = index("1") it's still a union...

[lament snow]

yes, because generics default to the constraint

if you want the default to be different then you need to provide your own default

[knotty stag]

or just use overloads

[lament snow]

ohhhh

i forgor

[knotty stag]

conditional returns/params almost never work

[lament snow]

yes overloads are safer

[knotty stag]

except for validator pattern

if you want to use conditionals in a signature in a way that isn't validation, you probably want overloads instead

[lament snow]

with overloads you wont accidentally explicitly provide the type even though you don't pass sk

[fleet prism]

sorry what you mean here?

[lament snow]

oh umm like, T extends (C ? A : B) should be true if T extends A and T extends B

[fleet prism]

Aah I see. Althoug this: T extends number ? T : Twas a bit different case because T might not extend number.

[lament snow]

but it doesn't matter right? because even if it doesn't extend, the resulting type is still T

because the false case is T

[fleet prism]

Yeah both result is T

That's what you meant

[lament snow]

yeah

[fleet prism]

But yeah these return types really don't work in most of the cases

with conditionals I mean

[lament snow]

yeah - i'm guessing that's the reason ? T : T doesn't work either

(because if both sides are the same then you might as well remove the entire conditional type completely)

[fleet prism]

but it shouldn't error imho

[fleet prism]

I see you mean the reason why it doesn't work is that T and T are same.

[lament snow]

nah

logically it should work because T and T are the same

so it doesn't matter whether the extends is true or not

(it shouldn't work in your original case which is ? 1 : 2, because typescript cannot tell that your function body is returning 1 in the appropriate cases)

[fleet prism]

I meant I was a bit confused here "yeah - i'm guessing that's the reason ? T : T doesn't work either", when you wrote "that's the reason", which reason you were referring to

[lament snow]

the reason = because they usually don't work, there's probably no point making it work in a very rare case that should never show up in real code

[fleet prism]

I see

[fleet prism]

In the original code I had I think as return type sk extends undefined? 1:2and you say if say sk was undefined, then return type would have been 1, and what if function returned 2 in that case right?

[lament snow]

yeah, exactly