Use socket.io inside a express routes file

I'm trying to use Socket.io with Node.js and emit to a socket within the logic of a route.

I have a fairly standard Express 3 setup with a server.js file that sits in the route, and then I have an index.js which sits in a routes folders that exports all the pages/publically accessible functions of the site. So they look like:

exports.index = function (req, res) {
    res.render('index', {
        title: "Awesome page"
    });
}; 

with the routing defined in server.js like:

app.get('/',routes.index);

I'm assuming I have to create the socket.io object in the server.js, since it needs the server object, but how can I access that object and emit to it from the index.js export functions?

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

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);

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');

      })

   });
}