var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.get('/', function(req, res){
  res.sendFile(__dirname + '/index.html');
});

io.on('connection', function(socket){
  console.log('a user connected');
});

http.listen(3000, function(){
  console.log('listening on *:3000');
});

<script src="/socket.io/socket.io.js"></script>
<script>
  var socket = io();
</script>

a user connected
a user connected
a user connected
...