If we have a type which is wrapped type like Promise. How we can get a type which is inside the wrapped type? For example if we have Promise<ExampleType> how to get ExampleType?
Take a look at below code. Write an utility type Transform which will take a generic type argument, and if it is a Promise it will evaluate to the type inside it.
type X = Promise<string>
type Y = Promise<{ field: number }>
type ResultX = Transform<X>; // ResultX type equals string
type ResultY = Transform<Y>; // ResultY type equals { field: number }
type Transform<A> = /** here your answer **/
Post your answers in comments. Have fun! Answer will be published soon!
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Top comments (16)
Nice little challenge, I wouldnt call it advanced tho, maybe more like intermediate?
Edit: heres another challenge: make a type Reverse which takes a tuple of any length and reverses its elements! (Hint: use the
{ 0: A; 1: B }[C extends D ? 0 : 1]hack to be able to use recursion in some cases where typescript woule've thrown an error)Nice challenge :) Here are 2 solutions
You are really picky :). I hope in the series I will put some questions which will satisfy you. Thanks for feedback.
Np, check the edit to the original comment to see a challenge idea
Nice one. Thank you!
Playground link. To avoid spoilers.
(On the phone so I re-wrote the example with Array)
I think the wrapper type cannot be generic. I mean the same solution wouldn't work for Promise and Array. Or at least I don't know a way.
Niiice :D
Yep, it's sad to see typescript missing so much from not having higher kinded types!
Can be simulated a bit though...
Wow, I was only familiar with the approach fp-ts took, its nice to see cool stuff like this:)
We can do a lot already, right? But yeah, it'd be nice to have.
I'm happy, I found your exercises! They are absolutely useful, exectly what I was looking for. A lot of thanks!
Ah, so this is where the
inferkeyword is useful:Basically, the
inferkeyword here lets us start referencing a new type, that didn't come from the original type declaration. In this case - we are returning it.This is mine. Nothing special :)
type Transform<A> = Awaited<Promise<A>>type Transform = A extends Promise ? R : A;
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