Luhn validation code in JavaScript (MooTools)

String.isValidLuhn.js

/*
 * Luhn algorithm number checker - (c) 2005-2008 shaman - www.planzero.org
 * Ported to Mootools by Chee Aun - http://cheeaun.com/
 * http://planzero.org/code/bits/viewcode.php?src=luhn_check.js
 * http://en.wikipedia.org/wiki/Luhn_algorithm
 */

String.implement({
	
	isValidLuhn: function(){
		var number = this.replace(/\D/g, ''); // strip non-digits
		var len = number.length;
		var parity = len % 2;
		var total = 0;
		
		for (i=0; i < len; i++) {
			var digit = number.charAt(i);
			// Multiply alternate digits by two
			if (i % 2 == parity){
				digit *= 2;
				// If the sum is two digits, add them together (in effect)
				if (digit > 9) digit -= 9;
			}
			// Total up the digits
			total += parseInt(digit);
		}

		// If the total mod 10 equals 0, the number is valid
		return (total % 10 == 0);
	}
	
});