how to get all permutations of an array

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}

1/* to be used something like this:
2int [] toBePermuted = new int [] {1, 2, 3, 4};
3ArrayList<int[]> a = heap(toBePermuted);
4any mention of int [] can be replaced with any other Array of objects */
5
6ArrayList<int []> heap(int [] input) {
7  ArrayList<int []> ret = new ArrayList<int []> ();
8  ret = generate(input.length, input, ret);
9  return ret;
10}
11
12ArrayList<int []> generate(int k, int [] a, ArrayList<int []> output) {
13  if (k == 1) {
14    output.add(a.clone());
15  } else {
16    output = generate(k-1, a, output);
17    for (int i=0; i<k-1; i++) {
18      if (k%2 == 0) {
19        int temp = a[i];
20        a[i] = a[k-1];
21        a[k-1] = temp;
22      } else {
23        int temp = a[0];
24        a[0] = a[k-1];
25        a[k-1] = temp;
26      }
27      generate(k-1, a, output);
28    }
29  }
30  return output;
31}