JavaScript Luhn validator method. Modifications made here include support for non-numeric characters in the value being cleaned up instead of rejecting the value. Also, cleaned up a duplicate variable and added identity check at the end instead of equality check.

validate_credit_card.js

// takes the form field value and returns true on valid number
function valid_credit_card(value) {

	// The Luhn Algorithm. It's so pretty.
	var nCheck = 0, bEven = false;
	//this will remove all non-numeric characters and validate against remaining code. 
	//therefore this now supports 4111-1111-1111-1111 patterns as an example
	value = value.replace(/\D/g, "");

	for (var n = value.length - 1; n >= 0; n--) {
		var cDigit = value.charAt(n),
			  nDigit = parseInt(cDigit, 10);

		if (bEven) {
			if ((nDigit *= 2) > 9) nDigit -= 9;
		}

		nCheck += nDigit;
		bEven = !bEven;
	}

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