Is there a way to define type for array with unique items in typescript?

Question

The type should detect if the array has duplicate items and throw error in typescript?

type UniqueArray = [
  // How to implement this?
]

const a:UniqueArray = [1, 2, 3] // success
const b:UniqueArray = [1, 2, 2] // error

PS: I'am currently removing duplicate items using JS, but, curious if this error can be captured using typescript type before hand?

Answer 1

Yes! There is a way with TypeScript 4.1 (in beta at the time of writing). This is how:

const data = ["11", "test", "tes", "1", "testing"] as const
const uniqueData: UniqueArray<typeof data> = data

type UniqueArray<T> =
  T extends readonly [infer X, ...infer Rest]
    ? InArray<Rest, X> extends true
      ? ['Encountered value with duplicates:', X]
      : readonly [X, ...UniqueArray<Rest>]
    : T

type InArray<T, X> =
  T extends readonly [X, ...infer _Rest]
    ? true
    : T extends readonly [X]
      ? true
      : T extends readonly [infer _, ...infer Rest]
        ? InArray<Rest, X>
        : false

You'll get a compiler error if the same value occurs more than once.

Here's my article describing things in better detail.

Try in TypeScript Playground