type-challenges-solutions/en/medium-return-type

[utterances-bot]

Get Return Type

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/medium-return-type.html

[zhaoyao91]

Why you fallback to T instead of never?

like this

type MyReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

[ghaiklor]

@zhaoyao91 there is no reason, it just a preferable choice for me.

[Brysonmk1984]

A couple of questions:

1. How is it valid that T extends (...args: any[]) covers the cases where no parameters are passed in? it seems like we would be passing it undefined rather than an empty array.

2. Regarding the previous comments, how is 'never' interchangeable with 'T'? Like what does it mean to return T here? and does returning 'never' just mean that if they try to use this type and pass in something other than a function, it will throw a TS error?

[ghaiklor]

@Brysonmk1984

how is 'never' interchangeable with 'T'?

It's not in this context. As I said, it was just a choice, not based on anything. Feel free to return never if you want.

does returning 'never' just mean that if they try to use this type and pass in something other than a function, it will throw a TS error?

never is a type that says some situation X must not happen. You can read more about it here