ws library for WebSocket server in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The ws library is used to create WebSocket servers and clients in Node.js, enabling real-time, two-way communication between a server and browser or other clients.

beginner

How do you create a basic WebSocket server using the ws library?

You create a WebSocket server by importing <code>WebSocketServer</code> from <code>ws</code> and then instantiating it with a port number, like:<br><pre>const { WebSocketServer } = require('ws');
const wss = new WebSocketServer({ port: 8080 });</pre>

beginner

What event do you listen to for new client connections on a ws WebSocket server?

You listen to the 'connection' event on the WebSocket server instance to handle new client connections.

beginner

How can you send a message to a connected client using the ws library?

You use the send() method on the client WebSocket object inside the 'connection' event handler, like ws.send('Hello client!').

beginner

Why is it important to handle the 'message' event on a WebSocket client connection?

Handling the 'message' event lets the server receive and process data sent from the client, enabling interactive real-time communication.

Which method creates a WebSocket server with the ws library?

A ws.createServer(8080)

B new WebSocketServer({ port: 8080 })

C new WebSocketClient({ port: 8080 })

D ws.startServer(8080)

The WebSocketServer class is used to create a WebSocket server instance.

What event should you listen to for new client connections?

A 'connection'

B 'client'

C 'open'

D 'connect'

The 'connection' event fires when a new client connects to the server.

How do you send a message to a client in ws?

A client.send(message)

B client.emit('send', message)

C server.send(client, message)

D ws.sendToClient(message)

The send() method on the client WebSocket object sends data to that client.

Which of these is NOT a valid use of the ws library?

A Creating a WebSocket server

B Creating a WebSocket client

C Handling HTTP requests

D Sending real-time messages

The ws library handles WebSocket connections, not HTTP requests.

What event do you listen to for receiving messages from a client?

A 'input'

B 'data'

C 'receive'

D 'message'

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

Explain how to set up a simple WebSocket server using the ws library in Node.js.

Think about the steps from starting the server to talking with clients.

Describe the role of the 'connection' and 'message' events in a ws WebSocket server.

Focus on how the server interacts with clients.

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