socketIONotes.js

// track individual socket status
var allClients = [];
io.sockets.on('connection', function(socket) {
   allClients.push(socket);

   socket.on('disconnect', function() {
      console.log('Got disconnect!');

      var i = allClients.indexOf(socket);
      allClients.splice(i, 1);
   });
});


const express = require('express');
const app = express();
     
// ...other middleware...
     
const server = app.listen(1337);
const io = require('socket.io')(server);
    
io.on('connection', function (socket) { 
  
  socket.on('alpha', function (data) { 
    // socket.emit will respond back to the socket client that triggered this 'alpha' listener
    socket.emit('updateClient', { data: 5 });
  });
  socket.on('beta', function (data) { 
    // io.emit will message all socket clients 
    io.emit('updateAllClients', { data: 5 });
  });
  socket.on('gamma', function (data) { 
    // socket.broadcast will message all socket clients except the one that triggered the 'gamma' listener
    socket.broadcast.emit('updateAllExceptOne', { data: 5 });
  });
});


const express = require('express');
const app = express();
app.use(express.static(__dirname + "/public"));
const server = app.listen(1337);
const io = require('socket.io')(server);
var counter = 0;
    
io.on('connection', function (socket) { //2
  
  socket.emit('greeting', { msg: 'Greetings, from server Node, brought to you by Sockets! -Server' }); //3
  socket.on('thankyou', function (data) { //7
    console.log(data.msg); //8 (note: this log will be on your server's terminal)
  });
    
});


<html>
<head>
    <title>Sockets</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
    <script type="text/javascript" src="/socket.io/socket.io.js"></script>
    <script type ="text/javascript">
        $(document).ready(function (){
    
            var socket = io(); //1
    
            socket.on('greeting', function (data) { //4
                console.log(data.msg); //5
                socket.emit('thankyou', { msg: 'Thank you for connecting me! -Client' }); //6
            });
         })
    </script>
</head>
<body>
    <h1>Fun with sockets</h1>  
</body>
</html>