permute-lists-fp.ts

type TupleToList<xs> = xs extends [infer head, ...(infer tail)]
  ? [head extends any[] ? head[number] : never, ...TupleToList<tail>]
  : [];

  
  
function permute<xs extends [] | [any[], ...any[][]]>([
  head,
  ...rest
]: xs): TupleToList<xs>[];
function permute<T>([head, ...rest]: T[][]): T[][];
function permute<T>([head, ...rest]: T[][]): T[][] {
  if (!head) return [[]];
  if (!rest.length) return head.map((x) => [x]);
  return head.flatMap((item) => permute(rest).map((perm) => [item, ...perm]));
}


console.log(permute([])); // -> []
console.log(permute([[1, 2, 3]]));  // -> [[1], [2], [3]]
console.log(
  permute([
    [1, 2, 3],
    ["a", "b"]
  ])
);   // -> [[1, "a"], [1, "b"], [2, "a"], [2, "b"], [3, "a"], [3, "b"]]

console.log(
  permute([
    [1, 2, 3],
    ["a", "b", "c"],
    [true, false]
  ])
);