// noprotect

function swap(arr, a,b){

  var temp = arr[a];

  arr[a]=arr[b];

  arr[b]=temp;

}

​

function factorial(n) {

  var val = 1;

  for (var i=1; i<n; i++) {

    val *= i;

  }

  return val;

}

​

​

function permute(perm, func){

  var total = factorial(perm.length);

  for (var j=0, i=0, inc=1;  j<total;  j++, inc*=-1, i+=inc) {

    for (; i<perm.length-1 && i >=0; i+=inc) {

      func.call(perm);

      swap (perm, i, i+1);

    }  

    func.call(perm);

    if (inc === 1) {

      swap(perm, 0,1);

    } else {

      swap(perm, perm.length-1, perm.length-2);

    }

  }

}

​

console.clear();

​

count = 0;

permute([1,2,3,4,5,6], function(){console.log(this); count++;});

​

console.log('There have been ' + count + ' permutations');