Factorial and BigNumbers

bignumber.js

var factorial = function(n) {
  var res = [1];
  var res_size = 1;

  for (var x = 2; x <= n; x++) {
    res_size = multiply(x, res, res_size);
  }
  return res.reverse();
};

var multiply = function(x, res, res_size) {
  var carry = 0;

  for (var i = 0; i < res_size; i++) {
    var prod = res[i] * x + carry;
    res[i] = prod % 10;
    carry = parseInt(prod / 10);
  }
  while (carry) {
    res[res_size] = carry % 10;
    carry = parseInt(carry / 10);
    res_size++;
  }
  return res_size;
};

console.log(factorial(25).join(''));