Server file for Private chat using node.js and socket.io

app.js

var app = require('express').createServer()
var io = require('socket.io').listen(app);
var fs = require('fs');

app.listen(8008);

// routing
app.get('/', function (req, res) {
  res.sendfile(__dirname + '/chat.html');
});

// usernames which are currently connected to the chat
var usernames = {};

function check_key(v)
{
	var val = '';
	
	for(var key in usernames)
    {
		if(usernames[key] == v)
		val = key;
	}
	return val;
}

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

	// when the client emits 'sendchat', this listens and executes
	socket.on('sendchat', function (data) {
		// we tell the client to execute 'updatechat' with 2 parameters
		io.sockets.emit('updatechat', socket.username, data);
	});

	// when the client emits 'adduser', this listens and executes
	socket.on('adduser', function(username){
		// we store the username in the socket session for this client
		socket.username = username;
		// add the client's username to the global list
		usernames[username] = socket.id;
		// echo to client they've connected
		socket.emit('updatechat', 'SERVER', 'you have connected');
		// echo to client their username
		socket.emit('store_username', username);
		// echo globally (all clients) that a person has connected
		socket.broadcast.emit('updatechat', 'SERVER', username + ' has connected: ' + socket.id);
		// update the list of users in chat, client-side
		io.sockets.emit('updateusers', usernames);
	});

	// when the user disconnects.. perform this
	socket.on('disconnect', function(){
		// remove the username from global usernames list
		delete usernames[socket.username];
		// update list of users in chat, client-side
		io.sockets.emit('updateusers', usernames);
		// echo globally that this client has left
		socket.broadcast.emit('updatechat', 'SERVER', socket.username + ' has disconnected');
	});
	
	// when the user sends a private msg to a user id, first find the username
	socket.on('check_user', function(asker, id){
		//console.log("SEE: "+asker); console.log(id);
		io.sockets.socket(usernames[asker]).emit('msg_user_found', check_key(id));
	});
	
	// when the user sends a private message to a user.. perform this
	socket.on('msg_user', function(usr, username, msg) {
		//console.log("From user: "+username);
		//console.log("To user: "+usr);
		//console.log(usernames);
		io.sockets.socket(usernames[usr]).emit('msg_user_handle', username, msg);

		fs.writeFile("chat_data.txt", msg, function(err) {
			if(err) {
			console.log(err);
			} /*else {
			console.log("The file was saved!");
			}*/
			});
	});


});