JavaScript Factorial using strings for larger calculations. (Can get quite slow)

gistfile1.txt

var add = function(strNumOne, strNumTwo)
{
  var first;
	var second;

	var tempNumber = new Array();

	if(strNumOne.length >= strNumTwo.length)
	{
		first = strNumOne;
		second = strNumTwo;
	}
	else
	{
		first = strNumTwo;
		second = strNumOne;
	}

	var count = 0;

	var difference = first.length - second.length;

	var carryOver = 0;

	for (var i = second.length - 1; i >= 0; i--) 
	{
		var temp = parseInt(second[i]) + parseInt(first[i+difference]) + carryOver;

		if (temp >= 10)
		{
			temp = temp - 10;
			carryOver = 1;
		}
		else
		{
			carryOver = 0;
		}

		tempNumber[count] = temp;
		count++;
	};




	for (var i = difference; i > 0; i--) 
	{
		var temp = parseInt(first[i-1]) + carryOver;

		if (temp >= 10)
		{
			temp = temp - 10;
			carryOver = 1;
		}
		else
		{
			carryOver = 0;
		}

		tempNumber[count] = temp;

		count++;
	};

	if (difference === 0 && carryOver != 0)
	{
		tempNumber[count] = carryOver;
	}

	var finalNumber = new Array();
	for (var i = tempNumber.length - 1; i >= 0; i--)
	{

		finalNumber += String(tempNumber[i]);
	};

	return finalNumber;
}

var multiply = function(strNumOne, strNumTwo)
{
	var first;
	var second;

	var tempNumber = new Array();

	if(strNumOne.length >= strNumTwo.length)
	{
		first = strNumOne;
		second = strNumTwo;
	}
	else
	{
		first = strNumTwo;
		second = strNumOne;
	}

	

	var difference = first.length - second.length;

	var tempNumberArray = ['0'];

	var countZeros = 0;

	for (var i = second.length - 1; i >= 0; i--)
	{
		var count = countZeros;
		for (var j = first.length - 1; j >= 0; j--) 
		{
			var temp = parseInt(second[i]) * parseInt(first[j]);
			for (var k = 0; k < count; k++) 
			{
				temp += '0';
			};
			tempNumberArray.push(temp);
			count++;
		};

		countZeros++;
		
	};

	var finalNumber = "0";

	for (var i = tempNumberArray.length - 1; i >= 0; i--) {
		finalNumber = (add)(String(finalNumber), String(tempNumberArray[i]));
	};

	return finalNumber;
}

var factorial = function(number)
{
	var factorialNumber = 1;
	function factorialMultiply(currentMultiply, currentNumber)
	{
		if(currentMultiply === '0')
		{
			currentMultiply = 1;
		}
		return (multiply)(String(currentMultiply), String(currentNumber))
	}

	for (var i = number; i >= 0; i--) {
		factorialNumber = (factorialMultiply)(String(i), String(factorialNumber));
	}
	return factorialNumber;
}