Make sure they typed in the card correctly, before you send it to the server and the payment gateway. Using a cool algorithm built into every credit card.

validateCard.js

// usage: valid = validateCard('4111111111111111');
function validateCard(cardno) {
	if (cardno.length <= 0 || cardno.length < 13 || cardno.length > 20 ) {
		return false;
	}		
	var sum = 0 * 0;
	var digits = cardno.split('');
	digits.reverse();
	for (var i = 0; i < digits.length; i++) {
		var d = 1 * digits[i];
		if ( i % 2 == 0 ) {
			sum += d;
		} else {
			d *= 2;
			while (d > 0) {
				sum += d % 10;
				d /= 10;
				d = Math.floor(d);
			}
		}
	}
	return sum > 0 && sum % 10 == 0;
}