296 - Permutation

[1wkk]

/* _____________ Your Code Here _____________ */

type Permutation<T, P = T> = [T] extends [never]
  ? []
  : P extends P
  ? [P, ...Permutation<Exclude<T, P>>]
  : never

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type U = 'A' | 'B' | 'C'

type cases = [
  Expect<Equal<Permutation<'A'>, ['A']>>,
  Expect<
    Equal<
      Permutation<'A' | 'B' | 'C'>,
      | ['A', 'B', 'C']
      | ['A', 'C', 'B']
      | ['B', 'A', 'C']
      | ['B', 'C', 'A']
      | ['C', 'A', 'B']
      | ['C', 'B', 'A']
    >
  >,
  Expect<
    Equal<
      Permutation<'B' | 'A' | 'C'>,
      | ['A', 'B', 'C']
      | ['A', 'C', 'B']
      | ['B', 'A', 'C']
      | ['B', 'C', 'A']
      | ['C', 'A', 'B']
      | ['C', 'B', 'A']
    >
  >,
  Expect<Equal<Permutation<boolean>, [false, true] | [true, false]>>,
  Expect<Equal<Permutation<never>, []>>
]

[1wkk]

• https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types learn how Union type works
• https://www.typescriptlang.org/docs/handbook/utility-types.html#excludeuniontype-excludedmembers learn Exclude

[1wkk]

• Exclude<'a', 'a'> -> never
• T = never, but T extends never not work, use [T] extends [never]