Socket.io integration with Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a client connects?

Given this Express and Socket.io server setup, what will the server log when a client connects?

Express

const express = require('express');
const http = require('http');
const { Server } = require('socket.io');

const app = express();
const server = http.createServer(app);
const io = new Server(server);

io.on('connection', (socket) => {
  console.log('A user connected');
  socket.on('disconnect', () => {
    console.log('A user disconnected');
  });
});

server.listen(3000);

A The server logs 'connection error' when a client connects.

B The server logs 'A user disconnected' immediately when a client connects.

C The server logs nothing when a client connects.

D The server logs 'A user connected' immediately when a client connects.

💡 Hint

Look at the event name inside io.on() and what it logs.

The connection event triggers when a client connects, so the server logs 'A user connected'.

📝 Syntax

intermediate

Which option correctly initializes Socket.io with Express?

Choose the correct way to integrate Socket.io with an Express app.

A const io = new Server(server); // where server is created by http.createServer(app)

B const io = new Server(app);

C const io = new Server(http.createServer(app));

D const io = new Server(); io.attach(app);

💡 Hint

Socket.io needs the HTTP server, not just the Express app.

Socket.io must be attached to the HTTP server created from Express, not the Express app itself.

❓ state_output

advanced

What is the value of 'count' after 3 clients connect and disconnect?

Consider this code snippet:

let count = 0;
io.on('connection', (socket) => {
  count++;
  socket.on('disconnect', () => {
    count--;
  });
});

What is the value of count after 3 clients connect and then all disconnect?

A 3

B 0

C -3

D 1

💡 Hint

Think about how count changes on connection and disconnection.

Each connection increments count by 1, and each disconnect decrements it by 1. After 3 connect and 3 disconnect events, count returns to 0.

🔧 Debug

advanced

Why does this Socket.io server not receive client messages?

Review this server code snippet:

io.on('connection', (socket) => {
  socket.on('message', (msg) => {
    console.log('Message:', msg);
  });
});

Clients send messages with socket.emit('msg', 'hello'). Why does the server not log the messages?

A The client must use <code>socket.on</code> to send messages.

B The server must use <code>io.emit</code> to receive messages.

C The server listens for 'message' but clients emit 'msg', so events don't match.

D The server needs to call <code>socket.connect()</code> before listening.

💡 Hint

Check the event names used by client and server.

Event names must match exactly. The server listens for 'message' but clients emit 'msg', so the server never receives those events.

🧠 Conceptual

expert

Which best describes how Socket.io manages real-time communication with Express?

Choose the statement that best explains Socket.io's integration with Express for real-time features.

A Socket.io attaches to the existing HTTP server created from Express to handle WebSocket and fallback transports alongside HTTP requests.

B Socket.io creates a separate HTTP server and proxies Express requests through it.

C Socket.io replaces Express's routing system to manage all client-server communication.

D Socket.io runs as a standalone server and requires clients to connect to a different port than Express.

💡 Hint

Think about how Socket.io works with the HTTP server Express uses.

Socket.io attaches to the HTTP server created from Express. It handles WebSocket connections and fallback transports on the same server and port as Express HTTP requests.