/*============================================================================*/

/*

This routine checks the credit card number. The following checks are made:

1. A number has been provided
2. The number is a right length for the card
3. The number has an appropriate prefix for the card
4. The number has a valid modulus 10 number check digit if required

If the validation fails an error is reported.

The structure of credit card formats was gleaned from a variety of sources on 
the web.

Parameters:
            cardnumber           number on the card
            cardname             name of card as defined in the card list below

Author:     John Gardner
Date:       1st November 2003
Updated:    26th Feb. 2005      Additional cards added by request

*/

/*
   If a credit card number is invalid, an error reason is loaded into the 
   global ccErrorNo variable. This can be be used to index into the global error  
   string array to report the reason to the user if required:
   
   e.g. if (!checkCreditCard (number, name) alert (ccErrors(ccErrorNo);
*/

var ccErrorNo = 0;
var ccErrors = new Array ()

ccErrors [0] = "Unknown card type";
ccErrors [1] = "No card number provided";
ccErrors [2] = "Credit card number is in invalid format";
ccErrors [3] = "Credit card number is invalid";
ccErrors [4] = "Credit card number has an inappropriate number of digits";

function checkCreditCard (cardnumber) {
   
  // Ensure that the user has provided a credit card number
  if (cardnumber.length == 0)  {
     alert('No card number provided');
     return false; 
  }
  
  // Check that the number is numeric, although we do permit a space to occur  
  // every four digits. 
  var cardNo = cardnumber
  var cardexp = /^([0-9]{4})\s?([0-9]{4})\s?([0-9]{4})\s?([0-9]{1,4})\s?([0-9]{0,4})$/;
  if (!cardexp.exec(cardNo))  {
     alert('Credit card number is in invalid format');
     return false; 
  }
    
  // Now remove any spaces from the credit card number
  cardexp.exec(cardNo);
  cardNo = RegExp.$1 + RegExp.$2 + RegExp.$3 + RegExp.$4 + RegExp.$5;
  // alert (cardNo); 
  // Now check the modulus 10 check digit
  
    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)  {
          alert('Credit card number is invalid');
          return false; 
    }
    
  
  // The credit card is in the required format.
  return true;
}

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