Node use socket.io with ejs and router - node.js

Question

I tried to get socket.io to run, but I always get a client error, so I think there is a logic bomb inside the code.
app.js
const express = require('express');
const app = express();
const router = express.Router();
var http = require('http').Server(app);
var io = require('socket.io')(http);
app.set('socketio', io);
const expressLayouts = require('express-ejs-layouts');
module.exports = router;
app.use(express.static(__dirname));
app.use(express.urlencoded({ extended: true }));
app.use(expressLayouts);
app.set('view engine', 'ejs');
app.use(function(req, res, next) {
next();
});
io.on('connection', function(socket){
console.log('a user connected');
});
app.use('/', require('./routes/test.js'));
const PORT = process.env.PORT || 8081;
app.listen(PORT, console.log(`Server started on port ${PORT}`));
Inside the route file I open the page
test.js
const express = require('express');
const router = express.Router();
// Dashboard
router.get('/start', (req, res, next) => {
res.render('index2', {
caseArray: 'eins'
});
});
module.exports = router;
And finally inside the template I added:
<script
src="https://code.jquery.com/jquery-3.3.1.slim.min.js"
integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo"
crossorigin="anonymous"
></script>
<script src="/socket.io/socket.io.js"></script>
<script>var socket = io();</script>
But I always get an error message inside the browser console:
GET http://localhost:8081/socket.io/socket.io.js net::ERR_ABORTED 404 (Not Found)
(index):24 Uncaught ReferenceError: io is not defined
at (index):24
As I understand from all the tutorial is, that node will send the socket.io within the request to the html page and that with the html page the server get connected. But there is no connection message because the files do not exists. Where is my problem? In client or server?

Answer 1

It seems node is trying to locate socket.io's js files on your server on the route /socket/socket.io.js which I assume you haven't defined
I would suggest you use Socket io's cdn link and place it between your html header tags
<script src='https://cdnjs.cloudflare.com/ajax/libs/socket.io/2.1.1/socket.io.js'></script>

Answer 2

For all with a similar problem. I solved it by using
server.listen(PORT, console.log(`Server started on port ${PORT}`));
instead of using
app.listen(PORT, console.log(`Server started on port ${PORT}`));