Javascript function determines whether a Credit Card number is "valid"

// The function determines whether a Credit Card number is "valid"
// Please note that a "valid" Credit Card number is not essentially a Credit Card in "Good Standing"

function isValidCreditCard(number) {
     if (number.indexOf("-")) {
          cc = number.split("-");
          number = "";
          for (var i = 0; i < cc.length; i++) number += cc[i];
     }

     // Another Version of what was performed above using String & Array Methods
     if (number.indexOf(" ")) {
          cc = number.split(" ");
          number = cc.join("");
     }

     // OR using RegExp we can combine the above two
     //  number = number.replace(/-|\s/g, "");

     /**********************************************/

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

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

     for (i = 0; i < l; i++) {
          digit = number.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--;
     }

     if ((sum % 10) == 0) return (true);
     else return (false);
}