TypeScript Permutations Type

Permutations.ts

/**
 * @document Permutations.ts
 *
 * I wanted to figure out, just for the challenge of it, whether I could, given an array type `A`, produce a type that
 * matches any array with every element of `A` exactly once in any order.  I *love* abusing the TS typing engine.  It
 * insulted my mother once.
 */

/** 
 * Returns an array type that includes every element of `T` exactly once in any order.
 *
 * Obviously this produces an `n!` number of matching arrays.  My computer took a few minutes to verify 9-length arrays.
 */
type Permutations<T extends readonly unknown[]> =
  T['length'] extends 0 | 1
    ? T // if T only has one permutation, just return it
    : {
      // put each member of T first in an array, and concatenate the permutations of T without that member
      [K in keyof T & number]: [T[K], ...Permutations<ExcludeElement<T, T[K]>>]
    }[keyof T & number]; // get the union of all permutations starting with each element of T

/** Removes the first instance of `T` from `A`. */
type ExcludeElement<A extends readonly any[], T extends any> =
  A extends readonly [infer H, ...infer R]
    ? H extends T ? T extends H
      ? R // we've found T; just return what's left
      : [H, ...ExcludeElement<R, T>] : [H, ...ExcludeElement<R, T>] // H is not our T
    : A;

type A = [1, 2, 3, 4, 5, 6, 7, 8, 9];
type X = Permutations<A>;
const x: X = [5, 1, 4, 2, 3, 6, 7, 9, 8];