/**
 * Heap's algorithm
 */

 function heapPermutation(arr,size,n){
  var swap = function (index1,index2) {
      var temp = arr[index1];
      arr[index1] = arr[index2];
      arr[index2] = temp;
  }
  if (size === 1){
      console.log(arr);
      return;
  }
  for (let i = 0; i < size; i++){
      heapPermutation(arr, size -1,n);

      swap(size % 2 ? 0 : i, size - 1);
  }
}

var arr = ['a','b','c','d'];

heapPermutation(arr,arr.length,arr.length);