Credit Card "Validation"

// 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"

// When users type their Credit Card number
// they usually use hyphens to separate the numbers into blocks
function isValidCreditCard(number) {
     if (number.indexOf("-")) { // checking to see if there is a hyphen
          // split out the numbers from the string which is delimited by the (-)
          // & put the individual number blocks into individual array elements
          // ie, number = "12-34", cc = number.split("-") => cc[0] == 12 & cc[1] == 34
          cc = number.split("-");
          number = ""; // "empty" the variable so that we can "reuse" it
          for (var i = 0; i < cc.length; i++) {
               number += cc[i]; // here we are rebuilding the string (number) without the delimiter (-)
          }
     }

     // instead of splitting on the (-) this time split on a space
     if (number.indexOf(" ")) {
          cc = number.split(" ");
          number = cc.join("");
     }

     // OR using RegExp we can combine the above two
     // replace either a "-" or a " " with an empty string globally
     // number = number.replace(/-|\s/g, "");

/***** The rest of the algorithm is beyond the scope of this course *****/

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);
}