Using Socket IO

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Server-side code:

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1) Setting up the sockets

Import Dependencies

const express = require("express");
var http = require("http");`
var socketIO = require("socket.io"); 

Initialize socket IO with server

const app = express();
var server = http.Server(app);
const io = socketIO(server); 

Start the server listening on port 8000

server.listen( 8000, function() {     
console.log("Server started!");
}

2) socket.on and socket.emit

Here, we just put stuff that handles a connection: To handle receiving:

socket.on("name_of_data", function (data) {  
    var proccessed_data = do_something_Function(data);
    socket.emit("new_data", processed_data); // Emit processed data back to client
};

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Client side code

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1) Starting socket on client and receiving from server:

var socket = io(); 

socket.on("new_data", function (data) { 
     console.log("Received data from the server! + data");
     var processed_data = some_processor_function(data);
     socket.emit("back_to_server", data);
}

2) Where to put this socket.on and socket.emit?

On the server: put them within the 'connection'

io.on('connection', newConnection); 

function newConnection(socket) {
    
    socket.on('cardPlayed', function broadcastCard(data) {
        // This function broadcasts the card to everyone 
        console.log('Received' + data.cardid + ' from: ' + data.username);
        socket.broadcast.emit('cardPlayed', data); // broadcast.emit will emit to ALL clients except the sender
    });  

More ways to use emit: Socket IO Emit Cheatsheet