Exercise 4

isCreditCard

function isCreditCard(CC) {
	if (CC.length > 19)
		return (false);

	var sum = 0; var mul = 1; var l = CC.length; var digit; var tproduct;
	
	for (var i = 0; i < l; i++) {
		
		digit = CC.substring(l - i - 1, l - i);
		tproduct = parseInt(digit, 10) * mul;
		
		sum = (tproduct >= 10) ? (sum + (tproduct % 10) + 1) : (sum + tproduct);
		mul = (mul == 1) ? mul+1 : mul-1;

	}
	
	return ((sum % 10) == 0);
	
}

console.log(isCreditCard('4111111111111111'));
console.log(isCreditCard('378282246310005'));
console.log(isCreditCard('5555555555554444'));