Emitting and receiving messages in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a message is emitted and received?

Consider an Express server using Socket.IO. When the server emits a message 'greet' with data 'Hello', what will the client receive?

Express

const io = require('socket.io')(3000);
io.on('connection', socket => {
  socket.emit('greet', 'Hello');
});

A The client receives the event 'greet' but with no data.

B The client receives the event 'message' with data 'Hello'.

C The client receives no event because emit is server-side only.

D The client receives the event 'greet' with data 'Hello'.

💡 Hint

Think about how Socket.IO sends named events with data.

The server emits an event named 'greet' with the string 'Hello'. The client listens for 'greet' and receives the data exactly as sent.

❓ state_output

intermediate

What is the value of the message count after multiple emits?

In an Express app with Socket.IO, the server counts how many times it emits a 'ping' message to a client. After emitting 3 times, what is the count?

Express

let count = 0;
const io = require('socket.io')(3000);
io.on('connection', socket => {
  for(let i=0; i<3; i++) {
    socket.emit('ping', 'ping');
    count++;
  }
});
// What is the value of count after connection?

A 3

B 0

C 1

D Undefined

💡 Hint

Count increments inside the loop for each emit.

The loop runs 3 times, each time emitting and incrementing count. So count is 3 after connection.

📝 Syntax

advanced

Which option correctly emits a message to all connected clients?

Given a Socket.IO server instance 'io', which code correctly emits 'update' with data 'refresh' to all clients?

Express

const io = require('socket.io')(3000);

A io.broadcast.emit('update', 'refresh');

B io.send('update', 'refresh');

C io.emit('update', 'refresh');

D io.toAll('update', 'refresh');

💡 Hint

Check the official Socket.IO method to emit to all clients.

The method io.emit sends the event to all connected clients. Other options are invalid or do not exist.

🔧 Debug

advanced

Why does the client not receive the emitted message?

Server code: const io = require('socket.io')(3000); io.on('connection', socket => { socket.emit('hello', 'world'); }); Client code: const socket = io('http://localhost:3000'); socket.on('Hello', data => { console.log(data); }); Why does the client not log anything?

A The server emits before client connects.

B The client listens for 'Hello' but the server emits 'hello' (case mismatch).

C The client does not connect to the correct port.

D The server emits to the wrong socket.

💡 Hint

Check event name spelling and case sensitivity.

Event names are case sensitive. 'hello' and 'Hello' are different events, so client never receives 'Hello'.

🧠 Conceptual

expert

What happens if you emit a message to a disconnected socket?

In an Express app using Socket.IO, if the server tries to emit an event to a socket that has disconnected, what is the expected behavior?

A The emit silently fails; no error is thrown but message is not delivered.

B The server automatically reconnects the socket and sends the message.

C The message queues until the socket reconnects.

D The server throws a runtime error stopping the app.

💡 Hint

Think about how Socket.IO handles disconnected clients.

Emitting to a disconnected socket does not throw errors. The message is simply not delivered because the client is gone.