type-challenges-solutions/en/easy-if

[utterances-bot]

If

This project is aimed at helping you better understand how the type system works, writing your own utilities, or just having fun with the challenges.

https://ghaiklor.github.io/type-challenges-solutions/en/easy-if.html

[AurelienWebnation]

Hey!
I found this solution too, but I don't have any compilation error (without adding extends boolean to the type parameter C). It's normal?

type If<C, T, F> = C extends true ? T : F;

[ghaiklor]

@AurelienWebnation maybe the type system became smart enough to figure it out? Try on the older versions of TypeScript if you are interested in getting an answer to this.

[shadaxv]

Such a solution will be more precise

type If<C extends boolean, T, F> = C extends true
  ? T
  : C extends false
  ? F
  : never;

[AurelienWebnation]

@shadaxv I think your C extends false condition is not necessary because this type parameter is constrained to boolean. So if C extends true, in the other case, C can only extends false.

[shadaxv]

This is correct for cases with a boolean type, but if you send another type and comment it out with // @ts-expect-error, the returned type will be false instead of never, but maybe I'm just over-thinking it and it's not needed

[fabiangambetta]

Hi, using this type IF<T, K, L> = T extends true? K: L;
I don't understand why with null the result is "true"
type fromNull = IF<null, "true", "false">; //true

[shadaxv]

Hi, using this type IF<T, K, L> = T extends true? K: L; I don't understand why with null the result is "true" type fromNull = IF<null, "true", "false">; //true

You have something wrong, because the code you showed will return "false"