title: sending message to specific id in socket.io
body: <p>this is the example code from official socket.io page about namespaces.</p>
  
  <pre><code>io.on('connection', function(socket){
    socket.on('say to someone', function(id, msg){
      socket.broadcast.to(id).emit('my message', msg);
    });
  });
  </code></pre>
  
  <p>and my head is spinning how Id is sent from client side in 'say to someone' event what will be the unique id? how it is generated or we have to generate it??</p>