How to use ExpressJS and Socket.io on a same port?

It’s described on the socket.io github page (as @Golo stated in your comment):

var app = express()
  , server = require('http').createServer(app)
  , io = io.listen(server);

server.listen(80);

This works, I have it running.

Probably what Golo have forgotten is to change the listen from app.listen(80) to server.listen(80). I’ve struggled with this too until I realised my stupid mistake.