Refer: Medium - Tuple to Union

Describe

Implement a generic TupleToUnion<T> which covers the values of a tuple to its values union.

For example

code snippetCopytypescript
type Arr = ['1', '2', '3']

type Test = TupleToUnion<Arr> // expected to be '1' | '2' | '3'

Test Cases

code snippetCopytypescript
import { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<TupleToUnion<[123, '456', true]>, 123 | '456' | true>>,
  Expect<Equal<TupleToUnion<[123]>, 123>>,
]

Solution

code snippetCopytypescript
// my solution
type TupleToUnion<T extends any[]> = T extends [infer P, ...infer K]
  ? K['length'] extends 1
    ? P | K[0]
    : P | TupleToUnion<K>
  : never
type TupleToUnion<T extends any[]> = T[number]

// other solution
type TupleToUnion<T extends readonly unknown[]> = T extends [infer First, ...infer Rest]
  ? First | TupleToUnion<Rest>
  : never