Broadcasting messages in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when broadcasting a message to all connected clients?

Consider a NestJS WebSocket gateway that broadcasts a message to all connected clients using this.server.emit('message', data). What will the clients receive?

NestJS

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

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

  broadcastMessage(data: string) {
    this.server.emit('message', data);
  }
}

A All connected clients receive the 'message' event with the data.

B Only the client that sent the last message receives the broadcast.

C No clients receive the message because emit is not a broadcast method.

D Only clients in a specific room receive the message.

💡 Hint

Think about how emit works on the server instance in Socket.IO.

Using this.server.emit sends the event to all connected clients, broadcasting the message globally.

❓ state_output

intermediate

What is the state of connected clients after broadcasting?

In a NestJS WebSocket gateway, after calling this.server.emit('update', payload), what happens to the list of connected clients?

NestJS

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

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

  sendUpdate(payload: any) {
    this.server.emit('update', payload);
  }
}

A Clients are disconnected automatically after receiving the broadcast.

B The list of connected clients is cleared after broadcasting.

C The list of connected clients remains unchanged after broadcasting.

D Only clients that received the update remain connected.

💡 Hint

Broadcasting sends messages but does not affect client connections.

Broadcasting messages does not change the connection state of clients; all remain connected unless explicitly disconnected.

📝 Syntax

advanced

Which option correctly broadcasts a message to a specific room?

Given a NestJS WebSocket gateway, which code snippet correctly broadcasts a message only to clients in the room 'chat-room'?

NestJS

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

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

  broadcastToRoom(message: string) {
    // Which option is correct here?
  }
}

A this.server.emit('chat-room', 'message', message);

B this.server.to('chat-room').emit('message', message);

C this.server.in('chat-room').broadcast.emit('message', message);

D this.server.broadcast.to('chat-room').emit('message', message);

💡 Hint

Use the Socket.IO method to target a specific room.

The to method targets a room, and emit sends the event to that room's clients.

🔧 Debug

advanced

Why does broadcasting fail with 'this.server.broadcast.emit'?

In a NestJS WebSocket gateway, a developer tries to broadcast a message using this.server.broadcast.emit('event', data) but it fails. Why?

NestJS

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

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

  sendEvent(data: any) {
    this.server.broadcast.emit('event', data);
  }
}

A The server instance requires a namespace before using 'broadcast'.

B 'broadcast' is deprecated and replaced by 'emit' on the server.

C 'broadcast' only works inside client event handlers, not in the gateway class.

D 'broadcast' is not a property of the server instance; it exists on individual sockets.

💡 Hint

Consider where the 'broadcast' property is available in Socket.IO.

The broadcast property exists on socket instances to send to all except the sender. The server instance does not have 'broadcast'.

🧠 Conceptual

expert

How does NestJS handle broadcasting messages in clustered environments?

In a NestJS application running multiple instances behind a load balancer, what must be done to ensure WebSocket broadcasts reach all clients connected to any instance?

A Use a shared adapter like Redis adapter to synchronize broadcasts across instances.

B Broadcasting works automatically across instances without extra setup.

C Use HTTP polling instead of WebSockets to avoid broadcast issues.

D Configure each instance to broadcast only to its own clients; cross-instance broadcast is not possible.

💡 Hint

Think about how multiple server instances share state for WebSocket events.

In clustered setups, a shared adapter like Redis is required to propagate broadcast events to all instances so all clients receive messages.