React client receives two connections using Socket.io - node.js

Question

I am developing a simple app with SocketIO and I am encountering this problem. In the server I have the following code:
const httpServer = require('http').createServer();
const socketIO = require('socket.io');
const port = process.env.PORT_WS || 5001;
const io = socketIO(httpServer, { cors: { origin: '*' } });
io.on('connection', (socket) => {
console.log('Connected to socket');
socket.on('join-room', () => {
console.log('joined room')
});
});
httpServer.listen(port, () => {
console.log(`Listening on the port ${port}`);
});
In the client I have the following code:
import { io } from 'socket.io-client';
export default class SocketConnection {
constructor() {
this.initializeSocketConnection();
this.initializeSocketEvents();
}
initializeSocketConnection() {
console.log('I am here');
this.socket = io('ws://localhost:5001');
}
initializeSocketEvents() {
this.socket.on('connect', () => {
console.log('Socket connected');
});
}
}
I get in the console two Socket connected messages.
This is not a re-render issue because the I am here message is logged only once.
I am using socket.io version 4.0.1 both in the client and in the backend.

Answer 1

So this is happening because, in React Strict Mode, constructors are called two times. React seems to hide this. As the console.log('Socket connected'); is inside an "on" event, React has no way to "hide" this. Thus, 'I am here' is going to be shown once but 'Socket connected' is going to be shown twice.