Use socket.io inside a express routes file

There is a better way to do this now with Express 4.0.

You can use app.set() to store a reference to the io object.

Base configuration:

var app = require('express')();
var server = app.listen(process.env.PORT || 3000);
var io = require('socket.io')(server);
// next line is the money
app.set('socketio', io);

Inside route or middleware:

exports.foo = function(req,res){
    // now use socket.io in your routes file
    var io = req.app.get('socketio');
    io.emit('hi!');
}

Information about app.set() and app.get() is below:

app.set(name, value)

Assigns setting name to value. You may store any value that you want, but certain names can be used to configure the behavior of the server. These special names are listed in the app settings table.

Calling app.set('foo', true) for a Boolean property is the same as calling app.enable('foo'). Similarly, calling app.set('foo', false) for a Boolean property is the same as calling app.disable('foo').

Retrieve the value of a setting with app.get().

Source: https://expressjs.com/en/api.html#app.set

aarosil's answer was great, but I ran into the same problem as Victor with managing client connections using this approach. For every reload on the client, you'd get as many duplicate messages on the server (2nd reload = 2 duplicates, 3rd = 3 duplicates, etc).

Expanding on aarosil's answer, I used this approach to use the socket object in my routes file, and manage the connections/control duplicate messages:

Inside Server File

// same as aarosil (LIFESAVER)
const app = require('express')();
const server = app.listen(process.env.PORT || 3000);
const io = require('socket.io')(server);
// next line is the money
app.set('socketio', io);

Inside routes file

exports.foo = (req,res) => {

   let socket_id = [];
   const io = req.app.get('socketio');

   io.on('connection', socket => {
      socket_id.push(socket.id);
      if (socket_id[0] === socket.id) {
        // remove the connection listener for any subsequent 
        // connections with the same ID
        io.removeAllListeners('connection'); 
      }

      socket.on('hello message', msg => {
        console.log('just got: ', msg);
        socket.emit('chat message', 'hi from server');

      })

   });
}

You can set up your routes file as a function, and pass the Socket.IO object when requiring the file.

module.exports = function(io) {
  var routes = {};
  routes.index = function (req, res) {
    io.sockets.emit('payload');
    res.render('index', {
      title: "Awesome page"
    });
  };
  return routes;
};

Then require routes like this:

var express = require('express');
var app = express();
var http = require('http');
var server = http.createServer(app);
var io = require('socket.io').listen(server);
var routes = require('./routes')(io);

Super late addition here, but I wanted to access a socket in my routes and specifically wanted to broadcast a message after saving to the database. I used the answer provided by @aarosil to set/get the io object, sent the each client its socket id on connection, then used the socket id in the route to be able to use socket.broadcast.emit() instead of io.emit().

In server:

const io = require('socket.io')(server)
app.set('socketio', io)

io.on('connect', socket => {
  socket.emit('id', socket.id) // send each client their socket id
})

I send the socket id with each req and then I can do the following in my routes:

router.post('/messages', requireToken, (req, res, next) => {

  // grab the id from the request
  const socketId = req.body.message.socketId

  // get the io object ref
  const io = req.app.get('socketio') 

  // create a ref to the client socket
  const senderSocket = io.sockets.connected[socketId]

  Message.create(req.body.message)
    .then(message => {

      // in case the client was disconnected after the request was sent
      // and there's no longer a socket with that id
      if (senderSocket) {

        // use broadcast.emit to message everyone except the original
        // sender of the request !!! 
        senderSocket.broadcast.emit('message broadcast', { message })
      }
      res.status(201).json({ message: message.toObject() })
    })
    .catch(next)
})