Using socket.io in Express 4 and express-generator’s /bin/www

Here is how you can add Socket.io to a newly generated Express-Generator application:

1. Create a file that will contain your socket.io logic, for example socketapi.js:

socketapi.js:

const io = require( "socket.io" )();
const socketapi = {
    io: io
};

// Add your socket.io logic here!
io.on( "connection", function( socket ) {
    console.log( "A user connected" );
});
// end of socket.io logic

module.exports = socketapi;

2. Modify your bin/www launcher. There are two steps: requiring your Socket.io api and attaching the HTTP server to your socket.io instance right after creating the HTTP server:

bin/www:

/**
 * Module dependencies.
 */

var app = require('../app');
var debug = require('debug')('socketexpress:server');
var http = require('http');
let socketapi = require("../socketapi"); // <== Add this line

/**
 * Get port from environment and store in Express.
 */

var port = normalizePort(process.env.PORT || '3000');
app.set('port', port);

/**
 * Create HTTP server.
 */

var server = http.createServer(app);
socketapi.io.attach(server); // <== Also add this line

(...)

3. Then you just need to add the Socket.io client in your index.html. Add the following just before the </body> closing tag:

index.html

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

4. Finally you can start your Express server:

• Unix based: DEBUG=myapp:* npm start
• Windows: set DEBUG=myapp:* & npm start

Note 1

If for any reason you need access to your socket api in your app.js, then you could instead import your socket api in app.js, and re-export it:

app.js

var express = require('express');
var socketapi = require("./socketapi"); // <== Add this line
(...)
// You can now access socket.io through the socketapi.io object
(...)
module.exports = { app, socketapi }; // <== Export your app and re-export your socket API here

Then in your bin/www launcher, instead of importing your socket api on its own line, just import it along your app:

bin/www

var {app, socketapi} = require('../app'); // <== Import your app and socket api like this
(...)
var server = http.createServer(app);
socketapi.io.attach(server); // <== You still have to attach your HTTP server to your socket.io instance

Note 2
This answer was updated to work with the latest Express Generator (4.16 at time of writing) and latest Socket.io (3.0.5 at time of writing).