app.js

var port = process.env.PORT || 3000,
    http = require('http'),
    fs = require('fs'),
    html = fs.readFileSync('index.html');
    url = require('url');

// here we keep all the messages
var myLastMessage = [];

var server = http.createServer(function (req, res) {

   // dealing with POST request
    if (req.method === 'POST' && url.parse(req.url).pathname === "/chatroom/") {
        var body = '';

        // get the content of the body
        req.on('data', function(chunk) {
            body += chunk;
        });

        // when we are done with body
        req.on('end', function() {

            // get the id parameter from the url
            var query = url.parse(req.url).query;
            var id = query.replace('id=', '');

            // create the message object
            var result = {
                id: id,
                message: body, 
            }

            // set the flag for checking if there is another message with the same id
            var found = false;

            // look for the last message for given id, if found -> replace, 
            // not found then just add
            myLastMessage.find(function(element, index, array)  {
                if(element.id === id) {
                    found = true;
                    myLastMessage.splice(index, 1, result);
                }
            });

            // at this point the id is not found 
            if(!found) myLastMessage.push(result);

            res.writeHead(200, 'OK', {
                'Content-Type': 'application/json',
                'Access-Control-Allow-Origin': '*',
                'Access-Control-Allow-Headers': 'Origin, X-Requested-With, Content-Type, Accept'
            });
            res.end("\n\nSuccess");

        });  // end of req.on request
    }  else if (req.method === 'POST') {

            res.writeHead(400, 'Bad request', {
                'Content-Type': 'application/json',
                'Access-Control-Allow-Origin': '*',
                'Access-Control-Allow-Headers': 'Origin, X-Requested-With, Content-Type, Accept'
            });
            res.write(JSON.stringify({Error : "POST request not well formed."}));
            res.end();
    }   // end of POST request


    // dealing with GET request
    if (req.method === 'GET' && url.parse(req.url).pathname === "/chatroom/") {
    
            // get the id parameter from the url
            var query = url.parse(req.url).query;
            var id = query.replace('id=', '');

            var lastMessage = {};

             myLastMessage.find(function(element, index, array)  {
                if(element.id === id) {
                   lastMessage = element;
                }
            });

            res.writeHead(200, 'OK', {
                'Content-Type': 'application/json',
                'Access-Control-Allow-Origin': '*',
                'Access-Control-Allow-Headers': 'Origin, X-Requested-With, Content-Type, Accept'
            });
            res.write(JSON.stringify(lastMessage));
            res.end();

    
    } else if (req.method === 'GET') {
       
        res.writeHead(200);
        res.write(html);
        res.end();
    }
});

// Listen on port 3000, IP defaults to 127.0.0.1
server.listen(port);

// Put a friendly message on the terminal
console.log('Server running at http://127.0.0.1:' + port + '/');