How to check if type is undefined in Typescript Conditional Types?

Question

I currently have this type:

type ReturnType<T, E> = [T, E]

I 'm trying to make this type "smarter" by these rules:

If T is undefined, then the type would be [undefined, E]
If E is undefined, then the type would be [T]
T and E can't both be undefined

I'm trying to apply these rules using Typescript's Conditional Types.
Is this even possible to implement?

This is what I got so far:

type ReturnType<T, E> = 
    T extends undefined 
    ? E extends undefined ? never : [undefined, E]
    : E extends undefined ? [T] : never

Alex Wayne · Accepted Answer · 2021-04-15 22:09:49Z

6

What you seem to be missing is that you can do tests for both T and E at the same time with:

[T, E] extends [TypeA, TypeB] ? ... : ...

Putting that to use you can do:

type MyReturnType<T, E> = 
    [T, E] extends [undefined, undefined] ? never
    : T extends undefined ? [undefined, E]
    : E extends undefined ? [T]
    : [T, E]

type A = MyReturnType<undefined, undefined> // never
type B = MyReturnType<string, undefined> // [string]
type C = MyReturnType<undefined, number> // [undefined, number]
type D = MyReturnType<boolean, string> // [boolean, string]

Playground

answered Apr 15, 2021 at 22:09

Alex Wayne

188k52 gold badges328 silver badges363 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to check if type is undefined in Typescript Conditional Types?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related