app.js

var app = require('http').createServer(handler)
  , io = require('socket.io').listen(app)
  , fs = require('fs')

app.listen(3000);

var clients = {};

function handler (req, res) {
  fs.readFile(__dirname + '/index.html',
  function (err, data) {
    if (err) {
      res.writeHead(500);
      return res.end('Error loading index.html');
    }

    res.writeHead(200);
    res.end(data);
  });
}

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

  socket.on('add-user', function(data){
    clients[data.username] = {
      "socket": socket.id
    };
  });

  socket.on('private-message', function(data){
    console.log("Sending: " + data.content + " to " + data.username);
    if (clients[data.username]){
      io.sockets.socket(clients[data.username].socket).emit("add-message", data);
    } else {
      console.log("User does not exist: " + data.username); 
    }
  });

});