WebSocket protocol concept in Node.js - Mini Project: Build & Apply

Basic WebSocket Server with Node.js

📖 Scenario: You want to create a simple chat server that uses WebSocket protocol to allow real-time communication between clients and the server.

🎯 Goal: Build a basic WebSocket server using Node.js that accepts client connections, listens for messages, and sends back a confirmation message.

📋 What You'll Learn

Create a WebSocket server using the ws library

Set up a port number configuration variable

Listen for client connections and messages

Send a confirmation message back to the client

💡 Why This Matters

🌍 Real World

WebSocket servers are used in real-time applications like chat apps, live notifications, and online games where instant communication is needed.

💼 Career

Understanding WebSocket protocol and how to implement it with Node.js is valuable for backend developers working on interactive web applications.

1

Set up WebSocket server import and initial server

Write code to import the WebSocketServer class from the ws library and create a new WebSocket server instance called wss that listens on port 8080.

Node.js

const { WebSocketServer } = require('ws');
// Your code here

Hint

Use require('ws') to import and create a new WebSocketServer with the port option set to 8080.

2

Add a port configuration variable

Create a constant variable called PORT and set it to 8080. Then update the WebSocket server instance wss to use this PORT variable instead of the hardcoded number.

Node.js

const { WebSocketServer } = require('ws');
const PORT = 8080;
const wss = new WebSocketServer({ port: PORT });
// Your code here

Hint

Define const PORT = 8080; and use it in the WebSocketServer constructor.

3

Listen for client connections and messages

Add an event listener on wss for the 'connection' event with a callback function that takes a ws parameter. Inside this callback, add another event listener on ws for the 'message' event that receives a message parameter.

Node.js

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

wss.on('connection', (ws) => {
  // Your code here
});

Hint

Use wss.on('connection', (ws) => { ... }) and inside it ws.on('message', (message) => { ... }).

4

Send confirmation message back to client

Inside the message event listener, send a message back to the client using ws.send() with the text 'Message received'.

Node.js

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

wss.on('connection', (ws) => {
  ws.on('message', (message) => {
    // Send confirmation message here
  });
});

Hint

Use ws.send('Message received') inside the message event listener.

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