WebSocket protocol concept in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the WebSocket protocol?

WebSocket is a communication protocol that allows a two-way, full-duplex connection between a client and a server over a single, long-lived TCP connection.

beginner

How does WebSocket differ from HTTP?

Unlike HTTP, which is request-response and short-lived, WebSocket keeps the connection open, allowing real-time, continuous data exchange without repeated handshakes.

intermediate

What is the initial step to establish a WebSocket connection?

The client sends an HTTP upgrade request to the server asking to switch the protocol from HTTP to WebSocket, called the handshake.

beginner

What does 'full-duplex' mean in WebSocket communication?

Full-duplex means both client and server can send and receive messages independently at the same time without waiting for the other side.

beginner

Why is WebSocket useful for real-time applications?

Because it keeps the connection open and allows instant two-way data flow, WebSocket is great for chat apps, live updates, games, and other apps needing fast interaction.

What protocol does WebSocket start with before switching?

A FTP

B TCP

C SMTP

D HTTP

WebSocket begins with an HTTP handshake to upgrade the connection to WebSocket.

Which feature best describes WebSocket communication?

A Request-response only

B Full-duplex

C One-way streaming

D Stateless

WebSocket supports full-duplex communication, meaning both sides can send data simultaneously.

What happens after the WebSocket handshake is successful?

A A persistent connection is established

B Client sends HTTP requests repeatedly

C Connection closes immediately

D Server sends a file download

After handshake, a persistent, open connection allows continuous data exchange.

Which is NOT a typical use case for WebSocket?

A Static website hosting

B Real-time gaming

C Live chat applications

D Stock price updates

Static websites do not require real-time two-way communication, so WebSocket is unnecessary.

What does the WebSocket protocol use to keep the connection alive?

A Repeated HTTP requests

B Multiple UDP packets

C A single TCP connection

D Email notifications

WebSocket uses a single TCP connection kept open for continuous communication.

Explain how a WebSocket connection is established and why it is different from a normal HTTP connection.

Think about the initial request and what changes after the handshake.

Describe real-life scenarios where WebSocket is a better choice than HTTP and why.

Consider apps that need instant updates without refreshing.

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