WebSocket protocol concept in Node.js

Introduction

WebSocket lets your app talk to a server in real-time. It keeps the connection open so messages can flow back and forth instantly.

You want a chat app where messages appear instantly.

You need live updates like sports scores or stock prices.

You build a multiplayer game that needs fast communication.

You want to show notifications without refreshing the page.

Syntax

Node.js

const ws = new WebSocket('ws://example.com/socket');

ws.onopen = () => {
  ws.send('Hello Server!');
};

ws.onmessage = (event) => {
  console.log('Message from server:', event.data);
};

ws.onclose = () => {
  console.log('Connection closed');
};

Use ws:// for unencrypted and wss:// for secure connections.

Events like onopen, onmessage, and onclose handle connection states.

Examples

Connect securely using wss:// and send a message when connected.

Node.js

const ws = new WebSocket('wss://example.com/socket');

ws.onopen = () => {
  ws.send('Hi!');
};

Listen for messages from the server and log them.

Node.js

ws.onmessage = (event) => {
  console.log(`Received: ${event.data}`);
};

Detect when the connection closes and log it.

Node.js

ws.onclose = () => {
  console.log('Socket closed');
};

Sample Program

This Node.js program creates a WebSocket server on port 8080. When a client connects, it logs a message. It listens for messages from the client, logs them, and sends back the same message prefixed with 'Echo:'. It also logs when the client disconnects.

Node.js

import WebSocket from 'ws';

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

server.on('connection', (socket) => {
  console.log('Client connected');

  socket.on('message', (message) => {
    console.log('Received:', message.toString());
    socket.send(`Echo: ${message}`);
  });

  socket.on('close', () => {
    console.log('Client disconnected');
  });
});

console.log('WebSocket server running on ws://localhost:8080');

Important Notes

WebSocket keeps the connection open, unlike HTTP which closes after each request.

Use secure WebSocket (wss://) for sensitive data to protect privacy.

WebSocket works well for apps needing instant updates without refreshing.

Summary

WebSocket creates a live, two-way connection between client and server.

It is great for real-time apps like chat, games, and live feeds.

Use event handlers to send and receive messages easily.

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