非递归实现

function factorialize(num) {
  var result = 1;
    if(num < 0) return -1;
    if(num == 0 || num == 1) return 1;
    while(num>1) {
      result *= num--;
    }
    return result;
}

递归实现

function factorialize(num) {
  var result = 1;
  if(num < 0) return -1;
  if(num == 0 || num == 1) return 1;
  if(num > 1) return num*factorialize(num-1);
}