index.js

const express = require('express');
const socket = require('socket.io');

//App setup 
const app = express();
const server = app.listen(5000, function(){
    console.log('App listening on port 5000')
});

//Static files
app.use(express.static('public'));   //This will serve index.html file from the public folder to browser.

//Socket setup 
var io = socket(server)  //server because we want socket.io to work on this server which we created above.

//Listen to events (i.e handle the messages coming from the frontend)
io.on('connection', function(socket){      //listening for an event
    console.log('made socket connection', socket.id) ;

    socket.on('chat', function(data){   //When a message is sent form a client to the server. When the message with the name 'chat' comes from a client then the server will take the message
        io.sockets.emit('chat', data);  //and will emit that message to all the sockets(i.e all the clients) connected to the server.
    });

    socket.on('typing', function(data){    //When a message with the name 'typing' comes to the server 
        socket.broadcast.emit('typing', data)   //Broadcast the data(ie. handle.value) to all the clients except the one feom whom it came(i.e the one who is typing)
    })
})