Type-safe tuple composition from literal numbers

tuples.ts

/** The union of the types of members of array `Arr` */
export type MemberOf<Arr extends readonly unknown[]> = Arr[number];

// from https://stackoverflow.com/a/65914848/2257198
type Tuple<T, N extends number, A extends unknown[] = []> = A extends { length: N } ? A : Tuple<T, N, [...A, T]>;

export function composeTuple<Item, Length extends number, LengthCast = [...Item[]]>(item: Item, length: Length) {
  return Array.from({ length }).map(() => item) as LengthCast;
}

export function composeOneOrMore<Item, Length extends number>(item: Item, length: Length) {
  if (length < 1) {
    throw new Error(`Cannot create tuple of length ${length}`);
  }
  return composeTuple<Item, Length, Tuple<Item, Length, [Item]>>(item, length);
}

export function hasLength<A extends [...unknown[]], Length extends number>(
  array: A,
  length: Length,
): array is A & Tuple<MemberOf<A>, Length> {
  return array.length === length;
}