Socket.io overview in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Socket.io server code do?

Consider this Node.js Socket.io server snippet. What will happen when a client connects?

Node.js

import { Server } from 'socket.io';
const io = new Server(3000);
io.on('connection', (socket) => {
  socket.emit('welcome', 'Hello client!');
});

A The server sends 'Hello client!' to the connected client immediately.

B The server waits for the client to send a message before responding.

C The server broadcasts 'Hello client!' to all connected clients.

D The server closes the connection after sending a welcome message.

💡 Hint

Look at the 'socket.emit' inside the 'connection' event.

When a client connects, the server triggers the 'connection' event. Inside, it sends a 'welcome' message only to that client using socket.emit.

❓ state_output

intermediate

What is the output of this client-side Socket.io code?

Given this client code, what will be logged in the browser console?

Node.js

import { io } from 'socket.io-client';
const socket = io('http://localhost:3000');
socket.on('welcome', (msg) => {
  console.log(msg);
});

A An error will occur because the server URL is missing the port.

B Nothing will be logged because the event name is wrong.

C 'welcome' will be logged instead of the message.

D 'Hello client!' will be logged when the server sends it.

💡 Hint

Check the event name and server URL in the code.

The client listens for the 'welcome' event and logs the message sent by the server. The URL includes the port 3000, matching the server.

📝 Syntax

advanced

Which option correctly initializes a Socket.io server with CORS enabled?

You want to allow clients from any origin to connect. Which code snippet correctly sets up the server?

A const io = new Server(3000, { cors: { origin: '*' } });

B const io = new Server(3000, { cors: '*' });

C const io = new Server(3000, { cors: { origins: '*' } });

D const io = new Server(3000, { cors: { origin: true } });

💡 Hint

Check the correct property name for CORS origin in Socket.io options.

The correct way to enable CORS for all origins is to set cors: { origin: '*' }. Other options are invalid or incorrect property names.

🔧 Debug

advanced

Why does this Socket.io client fail to receive messages?

Client code: import { io } from 'socket.io-client'; const socket = io('http://localhost:3000'); socket.on('message', (data) => { console.log('Received:', data); }); Server code: import { Server } from 'socket.io'; const io = new Server(3000); io.on('connection', (socket) => { socket.emit('msg', 'Hello'); }); Why does the client never log 'Received: Hello'?

A The server emits before the client connects, so the message is lost.

B The client listens for 'message' but the server emits 'msg', so events don't match.

C The client forgot to call socket.connect() explicitly.

D The server is not running on port 3000.

💡 Hint

Check the event names used on client and server.

The client listens for 'message' events but the server emits 'msg'. Event names must match exactly for communication.

🧠 Conceptual

expert

What is the main advantage of using Socket.io over plain WebSockets?

Choose the best explanation for why developers prefer Socket.io instead of using raw WebSocket APIs.

A Socket.io requires no server setup and works purely on the client side.

B Socket.io uses less bandwidth than WebSockets by compressing all messages automatically.

C Socket.io automatically handles reconnection, fallback transports, and event-based messaging, making real-time apps easier to build.

D Socket.io only works with HTTP/2, which is faster than WebSocket.

💡 Hint

Think about features Socket.io adds beyond basic WebSocket communication.

Socket.io provides extra features like automatic reconnection, fallback to other protocols if WebSocket is unavailable, and easy event-based communication. This simplifies building real-time apps.