type-challenges-solutions/en/easy-first

[utterances-bot]

First of Array

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-first.html

[dvlden]

I was struggling to understand what does T extends [] refer to, so I asked on Type Challenge - Discord channel and I got pretty clear response.

Basically it extends empty array, if you try to set T extends unknown[] it fails, because it means that inside the array, we expect unknown items.

Here's my solution to avoid any[] and to clarify T extends []:

type First<T extends Array<unknown>> = T extends Array<never> ? never : T[0];

[likui628]

another solution:

type First<T extends any[]> = T extends [infer F, ...infer _] ? F : never

[anhuiliujun]

yet another solution:

type First<T extends any[]> = T['length'] extends 0 ? never : T[0]

[AurelienWebnation]

I was struggling to understand what does T extends [] refer to, so I asked on Type Challenge - Discord channel and I got pretty clear response.

Basically it extends empty array, if you try to set T extends unknown[] it fails, because it means that inside the array, we expect unknown items.

Here's my solution to avoid any[] and to clarify T extends []:

type First<T extends Array<unknown>> = T extends Array<never> ? never : T[0];

@dvlden I prefer this approach personnaly instead of using Array<> generic type!

type First<T extends unknown[]> = T extends [] ? never : T[0];

But I don't understand why you want to use unknown instead of any? In this case, I think using any is not a mistake, because we accept any type into our array, explicitly.

[LeeManh]

Solution use 'infer' :

type First<T extends any[]> = T extends [infer F, ...any[]] ? F : nerver

[AurelienWebnation]

Solution use 'infer' :

type First<T extends any[]> = T extends [infer F, ...any[]] ? F : nerver

@LeeManh Solution already provided by @likui628.

[gish]

This seems to work as well

type First<A> = A extends [infer T, ...any[]] ? T : never;