Credit card verification luhn checksum method

gistfile1.php

/**
 * Credit card verification luhn checksum method
 */
function validateLuhnChecksum(cardNumber) {
	if (cardNumber == 0) { return false; }
	var sum = 0;
	var mul = 1;
	for (var x = cardNumber.length; x > 0 ; x--) {
		var tproduct = parseInt(cardNumber.charAt(x - 1), 10) * mul;
		if (tproduct >= 10) {
			sum += (tproduct % 10) + 1;
		} else {
			sum += tproduct;
		}
		if (mul == 1) {
			mul++;
		} else {
			mul--;
		}
	}
	return ((sum % 10) == 0);
}