based off http://stackoverflow.com/questions/3959211/fast-factorial-function-in-javascript

factorial.js

if (typeof Math.factorial === "undefined") {

  Math.factorial = function factorial (n) {
    if (n == 0 || n == 1)
      return 1;
    if (Math.factorial.f[n] > 0)
      return Math.factorial.f[n];
    else
      return Math.factorial.f[n] = factorial(n-1) * n;
  };

  Math.factorial.f = [];

}