Unique array type

[brazen thunder]

I have a readonly array of IDs, and I am trying to write a type UniqueArray which would return the original array type if the are no repeated values, and never otherwise. I have tried different ways of creating such a type but had no success.

I hope someone who understands TypeScript better than me would be willing to help.

Reproduction below (noop).

[loud flame]

deminearchiver#0

Playground Link

Preview:```ts
// T if the array contains only unique strings, never otherwise
type UniqueArray<T extends readonly string[]> = T;

declare const ids1: ["one", "two", "three", "four", "five"];
declare const case1: UniqueArray<typeof ids1> // typeof ids1

declare const ids2: ["one", "one", "three", "two"];
...```

You can choose specific lines to embed by selecting them before copying the link.

[worn sluice]

i can think of a somewhat cursed way

!:u2t

[loud flame]

joshcena63#0

`!chensida:u2t`: Shortened Playground Link

type U2I<U> = (
  U extends U ? (arg: U) => 0 : never
) extends (arg: infer I) => 0
  ? I
  : never

// For homogeneous unions, it picks the last member
type OneOf<U> = U2I<
  U extends U ? (x: U) => 0 : never
> extends (x: infer L) => 0
  ? L
  : never

type U2T<U, L = OneOf<U>> = [U] extends [never]
  ? []
  : [...U2T<Exclude<U, L>>, L]```

that_guy977#0

Playground Link

Preview:```ts
...
type UniqueArray<T extends readonly string[]> = U2T<
T[number]

["length"] extends T["length"]
? T
: never
...```

[worn sluice]

maybe the other way around could be better to handle unions inside the array

[brazen thunder]

I don't even understand what the first one does

that's so cursed

[worn sluice]

U2T = union to tuple
U2I = union to intersection

[brazen thunder]

What's up with those function types?

[worn sluice]

even though it's possible to a degree with this, it still wouldn't handle cases where the array isn't fully known, which could come up in practice
this is something better left for the runtime to deal with

[brazen thunder]

It is fully known as far as I'm aware

Thank you so much for a quick answer!

[brazen thunder]

!resolved

[compact holly]

Can you not just use a Set?

[brazen thunder]

I am not trying to filter out unique at runtime, but to create a type that would error statically when I try to pass in duplicates.
Please the question carefully next time.

[compact holly]

I read the question

[worn sluice]

https://xyproblem.info is a thing, so sometimes just the content of the question isn't enough to see the intent

[brazen thunder]

The project is too complex to give any context