cleanup.js

function isCreditCard( CC ) {
     var sum, mul, l, digit, tproduct, i;

     if (CC.length > 19)
          return (false);

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

     for (i = 0; i < l; i++) {
          digit = CC.substring(l-i-1, l-i);
          tproduct = parseInt(digit, 10) * mul;

          sum += (tproduct >= 10) ? (tproduct % 10) + 1 : tproduct;

          (mul == 1) ? mul++ : mul--;
     }

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