Credit card prediction in Javascript

Question

I'm writing a tool that onkeydown will run the current value being entered in an input box to check to see if it matches a regex for one of the 4 major types of credit cards.

I feel like it kind of works, but it's flaky so I wanted to figure out what was causing it to give faulty response (e.g. sometimes it'll output 2 values instead of one). Is it because I need to set a flag variable before looping? Upon match of the correct card, I'm just returning from the loop through the object, so I thought that'd be sufficient enough...

The criteria for the regexes were pulled from this site:

Visa: ^4[0-9]{12}(?:[0-9]{3})?$ All Visa card numbers start with a 4. New cards have 16 digits. Old cards have 13.
MasterCard: ^5[1-5][0-9]{14}$ All MasterCard numbers start with the numbers 51 through 55. All have 16 digits.
American Express: ^3[47][0-9]{13}$ American Express card numbers start with 34 or 37 and have 15 digits.

Discover: ^6(?:011|5[0-9]{2})[0-9]{12}$ Discover card numbers begin with 6011 or 65. All have 16 digits.

$(function() {

var $cardNumber = $('#js-cardnumber');

var ccMap = {};

ccMap.cards = {
    'amex': '^3[47][0-9]{13}$',
    'discover': '^6(?:011|5[0-9]{2})[0-9]{12}$',
    'mastercard': '^5[1-5][0-9]{14}$',
    'visa': '^4[0-9]{12}(?:[0-9]{3})?$'
};


$cardNumber.keydown(function() {
for (var cardType in ccMap.cards) {
    if (ccMap.cards.hasOwnProperty(cardType)) {
        var regex = ccMap.cards[cardType];
        if (regex.match($(this).val())) {
            console.log(cardType);
            return;
        }
    }
}
});
});

Here's a fiddle

Credit card prediction in Javascript

1 Answers