app.js

var http = require('http');
var express = require('express');
var app = express();
var SerialPort = require("serialport").SerialPort;
var server = http.createServer(app).listen(3000);
var io = require('socket.io').listen(server);

app.use(express.static(__dirname + '/public'));

var serialport = new SerialPort("/dev/ttyACM0"); // replace this address with your port address
serialport.on('open', function(){
	// Now server is connected to Arduino
	console.log('Serial Port Opened');

	var lastValue;
	io.sockets.on('connection', function (socket) {
		//Connecting to client 
		console.log('Socket connected');
		socket.emit('connected');
		var lastValue;

		serialport.on('data', function(data){
			var key = data.toString();
			console.log(key);
			// if(lastValue !== key){
				socket.emit('data', key);
			// }
			// lastValue = key;
		});
	});
});