function checkEAN13(ean13)
{

  if (ean13==null || ean13.length != 13)
    return false;

    chksum = 0;
    for (i = 0; i < ean13.length-1; i++)
    {
      j = Number(ean13.charAt(i));
      if (i % 2 == 0)
         chksum +=j;
      else
         chksum +=3*j;
    }//for
    chksum = chksum % 10;
    chksum = (10 - chksum) % 10;
    chksum = chksum - Number(ean13.charAt(12));
    return chksum==0;
}