Flushing Buffer data in Node.JS

Question

Currently having a problem in a app.

My app is designed to open an RFID reader on a Raspberry PI. It proceeds to read the incoming RFID tags.

The code is as follows;

// Socket.io server details
var io = require('socket.io').listen(3000);
// Serialport plugin declared and made a serialport variable
var serialport = require("serialport");
var SerialPort = serialport.SerialPort;
// Variable containing technical USB port details
var serialPort = new SerialPort("/dev/ttyUSB0", {baudrate: 2400, parser: serialport.parsers.readline("\n")} , false); // this is the openImmediately flag [default is true]

io.sockets.on('connection', function (socket) {

        console.log('user connected');

        socket.on('ping', function (data) {

                serialPort.open(function () {

                        // Open notification
                        console.log('opening RFID scanner');

                        //Start listening
                       serialPort.on('data', function(data) {



                                     if (data.trim() !== '') {

                                        io.sockets.emit('pong', data);

                                        socket.disconnect();
                                    }
                       }); 
                });
        });
});

I open the app, and it works just fine. It constantly can read tags of a card. However; when I use a different card, Node.JS seems to use some kind of buffer.

An example;

I scan card A, return tag AAA. I scan card A, return tag AAA. I scan card B, return tag AAA. I scan card B, return tag AAA. I scan card B, return tag BBB (The one that was supposed to be returned).

This always seems to happen when changing tags.

Is there some kind of buffer in Node.JS, that stores a buffer of data that wasnt transmitted yet?

I'm aware of the .drain (callback), but how could I implement this in a proper way?

Answer 1

I don't completely understand the node-serialport library, but I did have a similar problem when receiving data.

Each time my data value changed, I would always received the previous value once before then receiving the correct value on the next 'data' chunk.

I solved it by using the "flush" method in the node-serialport API.

A crude way of doing it would be:

sp.flush(function(err,results){});

Try testing this right before you disconnect the socket connection.

 if (data.trim() !== '') {
      io.sockets.emit('pong', data);
      sp.flush(function(err,results){});
      socket.disconnect();
 }