is Credit Card

gistfile1.js

// taken from http://www.dreamincode.net/code/snippet154.htm
//
// Find the weaknesses, and clean up the code.
// Find at least 3 things you can make better!
//
// Test numbers (return true):
// 4111111111111111
// 378282246310005
// 5555555555554444

function isCreditCard(CC) {

	var sum = 0,
		mul = 1,
		l = CC.length;


	if ( l > 19 ) {
		return false;
	}


	
	for ( i = 0; i < l; i++ ) {
		
		digit = CC.substring( (l - i) - 1, l - i);
		tproduct = parseInt(digit, 10) * mul;
		
		if (tproduct >= 10) {
			sum += (tproduct % 10) + 1;
		} else {
			sum += tproduct;
		}
		if (mul == 1) {
			mul++;
		} else {
			mul--;
		}

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