Untitled

<html>
  <head>
    <title>ISBN Digit calculate</title>
  </head>
  <body>
    <script type="text/javascript">
      function CalculateEAN13Checksum(messageString,checksumDigit)
      {
       var csumTotal = 0; // The checksum working variable starts at zero

       // If the source message string is less than 12 characters long, we make it 12 characters
       if( messageString.value.length < 12 )
        {
        var holdString = '000000000000' + messageString.value;
        messageString.value = holdString.substring(holdString.length - 12, holdString.length);
        }

       // Calculate the checksum value for the message
       for( charPos = messageString.value.length - 1; charPos >= 0; charPos--)
        {
        if( charPos / 2 == parseInt(charPos/2) )
         csumTotal = csumTotal + (parseInt(messageString.value.substring(charPos,charPos+1)));
        else
         csumTotal = csumTotal + (3 * parseInt(messageString.value.substring(charPos,charPos+1)));
        }

       // Calculate the checksum digit
       var remainder = csumTotal - parseInt(csumTotal/10) * 10;
       if( remainder == 0 )
        checksumDigit.value = '0';
       else
        checksumDigit.value = 10 - remainder;
      }
    </script>

    <form name="FormEAN13">
      <b>Enter ISBN-13</b>:
       <input type="text" maxlength="12" size="12" name="MESSAGE">
       <input type="BUTTON" value="Calculate" onclick="CalculateEAN13Checksum(document.FormEAN13.MESSAGE,document.FormEAN13.CHECKDIGIT);">
      <b>Check Digit:</b>
       <input name="CHECKDIGIT" type="text" maxlength="1" size="1"><br>
    </form>
  </body>
</html>