Exercise for JS Master Class

gistfile1.txt

function isCreditCard( CC ) {
     if (CC.length > 19)
          return (false);

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