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.

Practice

1. What is the main purpose of the ws library in Node.js?

easy

A. To connect to SQL databases

B. To create WebSocket servers and clients for real-time communication

C. To handle HTTP requests and routing

D. To manage file uploads in Node.js applications

Solution

1. Step 1: Understand the role of ws

   The ws library is designed specifically for WebSocket communication, enabling real-time data exchange between server and client.

2. Step 2: Compare with other options

   Options A, B, and C relate to other Node.js tasks like file uploads, HTTP routing, and database connections, which are not the focus of ws.

3. Final Answer:

   To create WebSocket servers and clients for real-time communication -> Option B

4. Quick Check:

   WebSocket library = real-time communication [OK]

Hint: Remember: ws = WebSocket server/client library [OK]

Common Mistakes:

• Confusing ws with HTTP or database libraries
• Thinking ws handles file uploads
• Assuming ws manages REST API routing

2. Which of the following is the correct way to create a WebSocket server using the ws library?

easy

A. const server = new WebSocketServer({ port: 8080 });

B. const server = new WebSocket({ port: 8080 });

C. const server = new WebSocketServer.listen(8080);

D. const server = ws.createServer(8080);

Solution

1. Step 1: Recall the correct constructor usage

   The ws library uses WebSocketServer constructor with an options object, including the port number.

2. Step 2: Check each option's syntax

   const server = new WebSocketServer({ port: 8080 }); correctly uses new WebSocketServer({ port: 8080 }). Options B, C, and D use incorrect constructors or methods not defined in ws.

3. Final Answer:

   const server = new WebSocketServer({ port: 8080 }); -> Option A

4. Quick Check:

   Correct constructor syntax = const server = new WebSocketServer({ port: 8080 }); [OK]

Hint: Use new WebSocketServer({ port }) to create server [OK]

Common Mistakes:

• Using WebSocket instead of WebSocketServer for server creation
• Calling listen() method which does not exist
• Using non-existent ws.createServer() method

3. What will the following code output when a client sends the message 'Hello' to the server?

const { WebSocketServer } = require('ws');
const wss = new WebSocketServer({ port: 8080 });
wss.on('connection', ws => {
  ws.on('message', message => {
    console.log(`Received: ${message}`);
    ws.send('Hi client');
  });
});

medium

A. Nothing happens because no connection event is handled

B. Server logs 'Hi client' and sends 'Received: Hello' back

C. Server throws an error because message event is invalid

D. Server logs 'Received: Hello' and sends 'Hi client' back

Solution

1. Step 1: Understand event handling

   The server listens for 'connection' events, then listens for 'message' events on each connection. When a message arrives, it logs the message and sends a reply.

2. Step 2: Trace the message flow

   When client sends 'Hello', server logs 'Received: Hello' and sends 'Hi client' back to client.

3. Final Answer:

   Server logs 'Received: Hello' and sends 'Hi client' back -> Option D

4. Quick Check:

   Message event logs input and replies = Server logs 'Received: Hello' and sends 'Hi client' back [OK]

Hint: Message event logs input, ws.send replies [OK]

Common Mistakes:

• Confusing message event with connection event
• Thinking server sends back the received message
• Assuming message event is invalid

4. Identify the error in this WebSocket server code snippet:

const { WebSocketServer } = require('ws');
const wss = new WebSocketServer({ port: 3000 });
wss.on('connection', function(ws) {
  ws.on('message', function(msg) {
    ws.send(msg.toUpperCase);
  });
});

medium

A. WebSocketServer should be imported from 'websocket' package

B. The 'connection' event should be 'connect'

C. Using msg.toUpperCase without parentheses causes a runtime error

D. ws.send cannot send strings, only buffers

Solution

1. Step 1: Check method usage on message

   The code uses msg.toUpperCase without parentheses, so it passes the function itself instead of calling it.

2. Step 2: Understand the impact

   This causes a runtime error because ws.send expects a string or buffer, not a function reference.

3. Final Answer:

   Using msg.toUpperCase without parentheses causes a runtime error -> Option C

4. Quick Check:

   Method call missing () causes error = Using msg.toUpperCase without parentheses causes a runtime error [OK]

Hint: Always call string methods with () to avoid errors [OK]

Common Mistakes:

• Confusing event names 'connection' vs 'connect'
• Importing WebSocketServer from wrong package
• Thinking ws.send cannot send strings

5. You want to broadcast a message to all connected clients using the ws library. Which code snippet correctly sends 'Hello all!' to every client connected to the server?

const { WebSocketServer } = require('ws');
const wss = new WebSocketServer({ port: 8080 });

// Broadcast function here
function broadcast(message) {
  // ???
}

hard

A. wss.clients.forEach(client => { if (client.readyState === WebSocket.OPEN) client.send(message); });

B. wss.clients.map(client => client.send(message));

C. wss.broadcast(message);

D. wss.send(message);

Solution

1. Step 1: Understand how to access clients

   The wss.clients set contains all connected clients. We must iterate over it to send messages.

2. Step 2: Check client connection state before sending

   Only clients with readyState === WebSocket.OPEN can receive messages safely.

3. Step 3: Verify each option

   wss.clients.forEach(client => { if (client.readyState === WebSocket.OPEN) client.send(message); }); correctly uses forEach and checks readyState. wss.clients.map(client => client.send(message)); uses map which is not appropriate here. Options A and D use non-existent methods.

4. Final Answer:

   wss.clients.forEach(client => { if (client.readyState === WebSocket.OPEN) client.send(message); }); -> Option A

5. Quick Check:

   Broadcast = forEach + readyState check [OK]

Hint: Use forEach and check readyState before sending [OK]

Common Mistakes:

• Using map instead of forEach for side effects
• Calling non-existent broadcast or send on server
• Not checking client readyState before sending