1function getArrayMutations(arr, perms = [], len = arr.length) {
2  if (len === 1) perms.push(arr.slice(0))
3
4  for (let i = 0; i < len; i++) {
5    getArrayMutations(arr, perms, len - 1)
6
7    len % 2 // parity dependent adjacent elements swap
8      ? [arr[0], arr[len - 1]] = [arr[len - 1], arr[0]]
9      : [arr[i], arr[len - 1]] = [arr[len - 1], arr[i]]
10  }
11
12  return perms
13}

1const permutations = arr => {
2  if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
3  return arr.reduce(
4    (acc, item, i) =>
5      acc.concat(
6        permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
7          item,
8          ...val,
9        ])
10      ),
11    []
12  );
13};