Credit Card Number validation using JS

// Card Number = Credit Card number
// Card name = Card Type

function checkCreditCard (cardnumber, cardname) {
   
  // Array to hold the permitted card characteristics
  var cards = new Array();

  // Define the cards we support. You may add addtional card types as follows.
 
  //  Name:         As in the selection box of the form - must be same as user's
  //  Length:       List of possible valid lengths of the card number for the card
  //  prefixes:     List of possible prefixes for the card
  //  checkdigit:   Boolean to say whether there is a check digit
 
  cards [0] = {name: "Visa",
               length: "13,16",
               prefixes: "4",
               checkdigit: true};
  cards [1] = {name: "MasterCard",
               length: "16",
               prefixes: "51,52,53,54,55",
               checkdigit: true};
  cards [2] = {name: "DinersClub",
               length: "14,16",
               prefixes: "305,36,38,54,55",
               checkdigit: true};
  cards [3] = {name: "CarteBlanche",
               length: "14",
               prefixes: "300,301,302,303,304,305",
               checkdigit: true};
  cards [4] = {name: "AmEx",
               length: "15",
               prefixes: "34,37",
               checkdigit: true};
  cards [5] = {name: "Discover",
               length: "16",
               prefixes: "6011,622,64,65",
               checkdigit: true};
  cards [6] = {name: "JCB",
               length: "16",
               prefixes: "35",
               checkdigit: true};
  cards [7] = {name: "enRoute",
               length: "15",
               prefixes: "2014,2149",
               checkdigit: true};
  cards [8] = {name: "Solo",
               length: "16,18,19",
               prefixes: "6334,6767",
               checkdigit: true};
  cards [9] = {name: "Switch",
               length: "16,18,19",
               prefixes: "4903,4905,4911,4936,564182,633110,6333,6759",
               checkdigit: true};
  cards [10] = {name: "Maestro",
               length: "12,13,14,15,16,18,19",
               prefixes: "5018,5020,5038,6304,6759,6761",
               checkdigit: true};
  cards [11] = {name: "VisaElectron",
               length: "16",
               prefixes: "417500,4917,4913,4508,4844",
               checkdigit: true};
  cards [12] = {name: "LaserCard",
               length: "16,17,18,19",
               prefixes: "6304,6706,6771,6709",
               checkdigit: true};
             
  // Establish card type
  var cardType = -1;
  for (var i=0; i<cards.length; i++) {

    // See if it is this card (ignoring the case of the string)
    if (cardname.toLowerCase () == cards[i].name.toLowerCase()) {
      cardType = i;
      break;
    }
  }
 
  // If card type not found, report an error
  if (cardType == -1) {
     ccErrorNo = 0;
     return false;
  }
 
  // Ensure that the user has provided a credit card number
  if (cardnumber.length == 0)  {
     ccErrorNo = 1;
     return false;
  }
   
  // Now remove any spaces from the credit card number
  cardnumber = cardnumber.replace (/\s/g, "");
 
  // Check that the number is numeric
  var cardNo = cardnumber
  var cardexp = /^[0-9]{13,19}$/;
  if (!cardexp.exec(cardNo))  {
     ccErrorNo = 2;
     return false;
  }
     
  // Now check the modulus 10 check digit - if required
  if (cards[cardType].checkdigit) {
    var checksum = 0;                                  // running checksum total
    var mychar = "";                                   // next char to process
    var j = 1;                                         // takes value of 1 or 2
 
    // Process each digit one by one starting at the right
    var calc;
    for (i = cardNo.length - 1; i >= 0; i--) {
   
      // Extract the next digit and multiply by 1 or 2 on alternative digits.
      calc = Number(cardNo.charAt(i)) * j;
   
      // If the result is in two digits add 1 to the checksum total
      if (calc > 9) {
        checksum = checksum + 1;
        calc = calc - 10;
      }
   
      // Add the units element to the checksum total
      checksum = checksum + calc;
   
      // Switch the value of j
      if (j ==1) {j = 2} else {j = 1};
    }
 
    // All done - if checksum is divisible by 10, it is a valid modulus 10.
    // If not, report an error.
    if (checksum % 10 != 0)  {
     ccErrorNo = 3;
     return false;
    }
  }

  // The following are the card-specific checks we undertake.
  var LengthValid = false;
  var PrefixValid = false;
  var undefined;

  // We use these for holding the valid lengths and prefixes of a card type
  var prefix = new Array ();
  var lengths = new Array ();
   
  // Load an array with the valid prefixes for this card
  prefix = cards[cardType].prefixes.split(",");
     
  // Now see if any of them match what we have in the card number
  for (i=0; i<prefix.length; i++) {
    var exp = new RegExp ("^" + prefix[i]);
    if (exp.test (cardNo)) PrefixValid = true;
  }
     
  // If it isn't a valid prefix there's no point at looking at the length
  if (!PrefixValid) {
     ccErrorNo = 3;
     return false;
  }
   
  // See if the length is valid for this card
  lengths = cards[cardType].length.split(",");
  for (j=0; j<lengths.length; j++) {
    if (cardNo.length == lengths[j]) LengthValid = true;
  }
 
  // See if all is OK by seeing if the length was valid. We only check the length if all else was
  // hunky dory.
  if (!LengthValid) {
     ccErrorNo = 4;
     return false;
  };  
 
  // The credit card is in the required format.
  return true;
}