JavaScript Master Class - Exercise Clean Up

// 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, length = CC.length;

    if (length > 19)
          return (false);
    
     for (i = 0; i < length; i++)
     {
          var digit = CC.substring(length-i-1, length-i),
              tproduct = parseInt(digit ,10)*mul;
         
          sum += ((tproduct >= 10) ? ((tproduct % 10) + 1) : tproduct);
          mul == 1 ? mul++ : mul--;
     }
    
     return ((sum % 10) == 0);
}

document.body.innerHTML = isCreditCard('4111111111111111')