check javascript function performance

var cached = {};
function factorial(num) {
  if (num === 1)
		return 1;
	else if (cached[num] !== undefined) {
		return cached[num];
	}
	else {
		return num * factorial(num - 1);
	}
}

var start = new Date().getMilliseconds();
factorial(19000);
var end = new Date().getMilliseconds();
console.log(end - start);