How to use the crc.crc16xmodem function in crc

function calculateChecksum(payload) {
  // This code calculates CRC16-XModem checksum of payload
  // and returns it as Buffer in little-endian order.
  const checksum = Buffer.alloc(2);
  checksum.writeUInt16LE(crc.crc16xmodem(payload), 0);
  return checksum;
}

function isValid(message) {
  if (!Constants.equals(message.slice(4, 16))) return false;

  var checksum = message.readUInt16BE(message.length - 2);

  return checksum === crc(message.slice(16, -2));
}

target: target,

    data: data
  });

  debug(command.toString());

  try {
    content = command.message;
  } catch (err) {
    return callback(err);
  }

  var checksum = new Buffer(2);

  checksum.writeUInt16BE(crc(content), 0);

  var message = Buffer.concat([this.address, Constants, content, checksum]);

  this.socket.send(message, 0, message.length,
    this.port, this.gateway, callback);
};

)
  )

  var data = [
    f(ID_PAYLOAD_FORMAT, PAYLOAD_FORMAT_EMV_QRCPS_MERCHANT_PRESENTED_MODE),
    f(ID_POI_METHOD, amount ? POI_METHOD_DYNAMIC : POI_METHOD_STATIC),
    f(ID_MERCHANT_INFORMATION_BOT, serialize([
      f(MERCHANT_INFORMATION_TEMPLATE_ID_GUID, GUID_PROMPTPAY),
      f(targetType, formatTarget(target))
    ])),
    f(ID_COUNTRY_CODE, COUNTRY_CODE_TH),
    f(ID_TRANSACTION_CURRENCY, TRANSACTION_CURRENCY_THB),
    amount && f(ID_TRANSACTION_AMOUNT, formatAmount(amount))
  ]
  var dataToCrc = serialize(data) + ID_CRC + '04'
  data.push(f(ID_CRC, formatCrc(crc.crc16xmodem(dataToCrc, 0xffff))))
  return serialize(data)
}