WebSocket protocol concept in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a WebSocket server that listens on port 8080.

Node.js

import { WebSocketServer } from 'ws';
const wss = new WebSocketServer({ port: [1] });

Drag options to blanks, or click blank then click option'

A3000

B443

C8080

D80

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 80 or 443 without proper permissions or HTTPS setup.

Choosing a random port number not commonly used for WebSocket.

The WebSocket server listens on port 8080, which is a common port for WebSocket connections during development.

2fill in blank

medium

Complete the code to send a message to a connected client when the connection opens.

Node.js

wss.on('connection', function connection(ws) {
  ws.[1]('Hello Client!');
});

Drag options to blanks, or click blank then click option'

Asend

Breceive

Copen

Dconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'receive' which is not a method on the WebSocket object.

Confusing event names with methods.

The send method is used to send data to the connected client over the WebSocket.

3fill in blank

hard

Fix the error in the code to properly handle incoming messages from clients.

Node.js

wss.on('connection', function connection(ws) {
  ws.on('[1]', function message(data) {
    console.log('Received:', data);
  });
});

Drag options to blanks, or click blank then click option'

Amessage

Bconnect

Csend

Dopen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'open' or 'connect' which are not message events.

Confusing method names with event names.

The message event is emitted when the server receives data from a client.

4fill in blank

hard

Fill both blanks to broadcast a message to all connected clients.

Node.js

wss.clients.forEach(function each(client) {
  if (client.readyState === [1]) {
    client.[2]('Broadcast message');
  }
});

Drag options to blanks, or click blank then click option'

A1

B0

Csend

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using readyState 0 which means connecting, not open.

Using 'close' instead of 'send' to send messages.

Clients with readyState === 1 are open and ready to receive messages. Use send to send the message.

5fill in blank

hard

Fill all three blanks to close a client connection with a status code and reason.

Node.js

ws.[1]([2], '[3]');

Drag options to blanks, or click blank then click option'

Aclose

B1000

CNormal closure

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'send' instead of 'close' to close the connection.

Using an incorrect status code or missing the reason string.

The close method closes the connection. Code 1000 means normal closure, and the string explains the reason.

Practice

1. What is the main purpose of the WebSocket protocol in Node.js applications?

easy

A. To send HTTP requests from client to server

B. To create a live, two-way communication channel between client and server

C. To serve static files like HTML and CSS

D. To store data permanently on the server

Solution

1. Step 1: Understand WebSocket communication

   WebSocket allows a persistent connection where both client and server can send messages anytime.

2. Step 2: Compare with other options

   HTTP requests are one-way and short-lived, static files serving and data storage are unrelated to WebSocket's main role.

3. Final Answer:

   To create a live, two-way communication channel between client and server -> Option B

4. Quick Check:

   WebSocket = two-way live connection [OK]

Hint: WebSocket means live two-way chat between client and server [OK]

Common Mistakes:

• Confusing WebSocket with HTTP request-response
• Thinking WebSocket serves static files
• Assuming WebSocket stores data permanently

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

easy

A. const ws = new WebSocket(8080);

B. const ws = WebSocket.createServer(8080);

C. const ws = new WebSocket.Server({ port: 8080 });

D. const ws = WebSocket.listen(8080);

Solution

1. Step 1: Recall 'ws' library syntax

   The 'ws' library creates a server with new WebSocket.Server and a config object with port.

2. Step 2: Check each option

   Only const ws = new WebSocket.Server({ port: 8080 }); uses correct constructor and port option. Others use invalid methods or constructors.

3. Final Answer:

   const ws = new WebSocket.Server({ port: 8080 }); -> Option C

4. Quick Check:

   WebSocket.Server + port config = correct [OK]

Hint: Use new WebSocket.Server({ port }) to create server [OK]

Common Mistakes:

• Using WebSocket constructor instead of WebSocket.Server
• Calling non-existent methods like createServer or listen
• Passing port as a direct argument instead of in an object

3. Given this Node.js WebSocket server code snippet, what will be logged when a client sends the message 'hello'?

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

medium

A. Received: hello

B. pong

C. Received: pong

D. hello

Solution

1. Step 1: Analyze message event handler

   When the server receives a message, it logs 'Received: ' plus the message content.

2. Step 2: Identify logged output

   Client sends 'hello', so console logs 'Received: hello'. The server sends back 'pong' but that is not logged.

3. Final Answer:

   Received: hello -> Option A

4. Quick Check:

   Server logs received message text [OK]

Hint: Server logs 'Received: message' on message event [OK]

Common Mistakes:

• Confusing sent message with logged message
• Thinking server logs the reply it sends
• Mixing up client and server console outputs

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

const WebSocket = require('ws');
const wss = new WebSocket.Server({ port: 8080 });
wss.on('connection', ws => {
  ws.on('message', (msg) => {
    ws.send(msg.toUpperCase);
  });
});

medium

A. The port number must be a string, not a number

B. WebSocket.Server should be WebSocket.createServer

C. The 'message' event handler should be 'onMessage'

D. msg.toUpperCase should be called as a function: msg.toUpperCase()

Solution

1. Step 1: Check usage of toUpperCase

   toUpperCase is a function and must be called with parentheses to get the uppercase string.

2. Step 2: Verify other parts

   WebSocket.Server is correct, 'message' event is correct, port can be a number.

3. Final Answer:

   msg.toUpperCase should be called as a function: msg.toUpperCase() -> Option D

4. Quick Check:

   Call string methods with () [OK]

Hint: String methods need () to execute, not just reference [OK]

Common Mistakes:

• Forgetting parentheses on string methods
• Using wrong event names like 'onMessage'
• Confusing WebSocket server creation syntax

5. You want to build a Node.js WebSocket server that broadcasts any message received from one client to all connected clients except the sender. Which code snippet correctly implements this behavior?

hard

A. wss.on('connection', ws => {
ws.on('message', message => {
wss.clients.forEach(client => {
if (client !== ws && client.readyState === WebSocket.OPEN) {
client.send(message);
}
});
});
});

B. wss.on('connection', ws => {
ws.on('message', message => {
ws.send(message);
});
});

C. wss.on('connection', ws => {
ws.on('message', message => {
wss.send(message);
});
});

D. wss.on('message', message => {
wss.clients.forEach(client => client.send(message));
});

Solution

1. Step 1: Understand broadcasting logic

   Broadcast means sending message to all clients except the sender, checking if client connection is open.

2. Step 2: Analyze each option

   wss.on('connection', ws => { ws.on('message', message => { wss.clients.forEach(client => { if (client !== ws && client.readyState === WebSocket.OPEN) { client.send(message); } }); }); }); loops over all clients, skips sender, checks readyState, and sends message correctly. Others either send only to sender, call invalid methods, or listen on wrong event.

3. Final Answer:

   Option A code snippet correctly broadcasts to all except sender -> Option A

4. Quick Check:

   Broadcast = send to all except sender with readyState check [OK]

Hint: Use wss.clients.forEach and skip sender to broadcast [OK]

Common Mistakes:

• Sending message only back to sender
• Calling send on wss object instead of clients
• Not checking client.readyState before sending