Rooms and namespaces in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a client joins a room in a NestJS WebSocket Gateway?

Consider a NestJS WebSocket Gateway where clients can join rooms. What is the effect of calling client.join('room1') inside a message handler?

NestJS

import { WebSocketGateway, SubscribeMessage, MessageBody, ConnectedSocket } from '@nestjs/websockets';
import { Socket } from 'socket.io';

@WebSocketGateway()
export class ChatGateway {
  @SubscribeMessage('joinRoom')
  handleJoinRoom(@MessageBody() room: string, @ConnectedSocket() client: Socket) {
    client.join(room);
  }
}

A The client is removed from all other rooms automatically when joining a new room.

B The client is disconnected from the server immediately after joining the room.

C The client can no longer send messages to the server after joining the room.

D The client is added to the specified room and will receive messages sent to that room.

💡 Hint

Think about what rooms do in socket.io and how they affect message delivery.

Calling client.join('room1') adds the client to that room. This means the client will receive messages sent to that room. It does not disconnect the client or restrict sending messages.

📝 Syntax

intermediate

Which code correctly creates a namespace in a NestJS WebSocket Gateway?

In NestJS, how do you define a WebSocket Gateway that listens on a custom namespace /chat?

A

@WebSocketGateway({ path: '/chat' })
export class ChatGateway {}

B

@WebSocketGateway('/chat')
export class ChatGateway {}

C

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway {}

D

@WebSocketGateway({ url: '/chat' })
export class ChatGateway {}

💡 Hint

Check the official NestJS documentation for the correct option key to specify a namespace.

The namespace option is used to specify the namespace for the gateway. Using @WebSocketGateway({ namespace: '/chat' }) correctly sets the namespace.

🔧 Debug

advanced

Why does broadcasting to a room not reach clients in NestJS?

Given this code snippet, why might clients in the room not receive the broadcasted message?

NestJS

import { WebSocketGateway, SubscribeMessage, ConnectedSocket, WebSocketServer } from '@nestjs/websockets';
import { Socket, Server } from 'socket.io';

@WebSocketGateway()
export class ChatGateway {
  @WebSocketServer()
  server: Server;

  @SubscribeMessage('sendMessage')
  handleMessage(@ConnectedSocket() client: Socket, message: string) {
    this.server.to('room1').emit('message', message);
  }

  handleConnection(client: Socket) {
    client.join('room1');
  }
}

A The server property is never initialized, so broadcasting fails silently.

B Clients must call join again after sending a message to receive broadcasts.

C The room name 'room1' is invalid and must be prefixed with a slash.

D The emit method requires a callback to confirm delivery to the room.

💡 Hint

Check how the server property is assigned or initialized.

The server property is declared but never assigned. Without initializing it (e.g., using @WebSocketServer() decorator), this.server.to(...) is undefined, so no messages are sent.

❓ state_output

advanced

What is the number of clients in a room after multiple joins and leaves?

Assuming clients join and leave rooms as shown, how many clients remain in room42?

NestJS

import { WebSocketGateway, ConnectedSocket, SubscribeMessage } from '@nestjs/websockets';
import { Socket } from 'socket.io';

@WebSocketGateway()
export class RoomGateway {
  @SubscribeMessage('join')
  joinRoom(@ConnectedSocket() client: Socket) {
    client.join('room42');
  }

  @SubscribeMessage('leave')
  leaveRoom(@ConnectedSocket() client: Socket) {
    client.leave('room42');
  }
}

// Scenario:
// Client A joins room42
// Client B joins room42
// Client A leaves room42
// Client C joins room42

A 2

B 3

C 1

D 0

💡 Hint

Count clients currently in the room after all join and leave actions.

Clients A and B join, then A leaves, then C joins. So clients B and C remain in room42, total 2.

🧠 Conceptual

expert

How do namespaces and rooms differ in NestJS WebSocket gateways?

Which statement best describes the difference between namespaces and rooms in NestJS WebSocket gateways?

A Namespaces and rooms are interchangeable terms for grouping clients in NestJS gateways.

B Namespaces are separate communication channels identified by a path, while rooms are groups within a namespace for targeted messaging.

C Rooms are used to authenticate clients, namespaces handle message encryption.

D Rooms create new WebSocket connections, namespaces only group clients logically without new connections.

💡 Hint

Think about how socket.io organizes connections and message delivery.

Namespaces create separate endpoints (paths) for WebSocket connections. Rooms are subsets inside a namespace to group clients for sending messages to specific groups.