Function for outputting the factorial of a number

factorial.js

function factorial(num) {
  // If the number is less than 0, reject it.
  if (num < 0) {
    return "You can't do a factorial on negative numbers.";
  }
  // If the number is 0, its factorial is 1.
  else if (num == 0) {
    return 1;
  }
  var output = num;
  while (num-- > 2) {
    output *= num;
  }
  return output;
}