ws library for WebSocket server in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a client sends a message?

Consider this WebSocket server code using the ws library. What will the server send back to the client when the client sends the message "hello"?

Node.js

import WebSocket, { WebSocketServer } from 'ws';
const wss = new WebSocketServer({ port: 8080 });
wss.on('connection', function connection(ws) {
  ws.on('message', function message(data) {
    ws.send(`Received: ${data}`);
  });
});

A The client receives: "hello"

B The client receives: "Received: hello"

C The client receives: "message: hello"

D The client receives nothing

💡 Hint

Look at what the server sends back inside the message event handler.

The server listens for messages and sends back a string prefixed with "Received: ". So when the client sends "hello", the server replies with "Received: hello".

❓ lifecycle

intermediate

Which event triggers when a client disconnects?

In the ws WebSocket server, which event should you listen to detect when a client closes the connection?

Node.js

import WebSocket, { WebSocketServer } from 'ws';
const wss = new WebSocketServer({ port: 8080 });
wss.on('connection', function connection(ws) {
  // Which event here?
  ws.on('???', () => {
    console.log('Client disconnected');
  });
});

A 'close'

B 'disconnect'

C 'end'

D 'exit'

💡 Hint

Think about the standard WebSocket event for connection closure.

The 'close' event fires when the client disconnects. Other events like 'disconnect' or 'end' are not used in ws.

📝 Syntax

advanced

Which option correctly creates a WebSocket server on port 3000?

Choose the code snippet that correctly creates a WebSocket server listening on port 3000 using the ws library.

A const wss = new WebSocketServer({ listen: 3000 });

B const wss = new WebSocket.Server(3000);

C const wss = new WebSocketServer(port=3000);

D const wss = new WebSocketServer({ port: 3000 });

💡 Hint

Check the correct way to pass options to the WebSocketServer constructor.

The ws library uses new WebSocketServer({ port: 3000 }) to create a server. Other options are invalid syntax or wrong property names.

🔧 Debug

advanced

Why does this server not respond to client messages?

Look at this code snippet. The server does not send any response back to clients when they send messages. What is the cause?

Node.js

import WebSocket, { WebSocketServer } from 'ws';
const wss = new WebSocketServer({ port: 8080 });
wss.on('connection', function connection(ws) {
  ws.on('message', (data) => {
    ws.send('Got your message');
  });
});

// Client sends a message but gets no response.

A The server code is correct; the client likely closed the connection before response.

B The server does not listen for 'message' events correctly because the callback uses arrow function.

C The server is missing the 'open' event listener to start sending messages.

D The server is missing the 'connection' event listener on the WebSocketServer instance.

💡 Hint

Check if the server code listens to the right events and sends messages properly.

The server code is correct and sends a response on 'message'. If the client gets no response, it may have closed the connection or not handled the response properly.

❓ state_output

expert

How many clients are connected after these events?

Given this server code, how many clients are connected after two clients connect and one disconnects?

Node.js

import WebSocket, { WebSocketServer } from 'ws';
const wss = new WebSocketServer({ port: 8080 });
let count = 0;
wss.on('connection', function connection(ws) {
  count++;
  ws.on('close', () => {
    count--;
  });
});

// Two clients connect, then one disconnects.

A 0

B 2

C 1

D 3

💡 Hint

Count increments on connection and decrements on close.

Two clients connect: count = 2. One disconnects: count = 1. So one client remains connected.