Gateway decorator in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a client connects to this gateway?

Consider this NestJS WebSocket gateway code. What will be logged when a client connects?

NestJS

import { WebSocketGateway, OnGatewayConnection } from '@nestjs/websockets';

@WebSocketGateway()
export class MyGateway implements OnGatewayConnection {
  handleConnection(client: any, ...args: any[]) {
    console.log('Client connected:', client.id);
  }
}

A Logs 'Client connected:' followed by the client's id

B Does not log anything because OnGatewayConnection is not implemented correctly

C Logs 'Client connected:' but client.id is undefined

D Throws a runtime error because handleConnection is missing a return type

💡 Hint

Check how the handleConnection method works in a gateway implementing OnGatewayConnection.

The handleConnection method is called automatically when a client connects. The client object has an id property, so it logs the message with the id.

📝 Syntax

intermediate

Which option correctly applies the Gateway decorator with a namespace?

Choose the correct way to define a WebSocket gateway with the namespace '/chat'.

A

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

B

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

C

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

D

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

💡 Hint

Remember the Gateway decorator accepts an options object for namespace.

The @WebSocketGateway decorator accepts an options object where namespace is specified as a property. Passing a string directly sets the port, not the namespace.

❓ lifecycle

advanced

What happens if you implement OnGatewayDisconnect but forget to add the decorator?

Given this gateway class implementing OnGatewayDisconnect but missing the @WebSocketGateway() decorator, what is the behavior when a client disconnects?

NestJS

import { OnGatewayDisconnect } from '@nestjs/websockets';

export class MyGateway implements OnGatewayDisconnect {
  handleDisconnect(client: any) {
    console.log('Client disconnected:', client.id);
  }
}

A A runtime error occurs because the decorator is missing

B The disconnect handler is called normally and logs the client id

C The disconnect handler is never called because the class is not registered as a gateway

D The server crashes on startup due to missing decorator

💡 Hint

Think about what the decorator does in NestJS gateways.

Without the @WebSocketGateway() decorator, NestJS does not recognize the class as a gateway, so lifecycle hooks like handleDisconnect are not called.

🔧 Debug

advanced

Why does this gateway fail to emit events to clients?

Examine the code below. Why does the sendMessage method not send messages to connected clients?

NestJS

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

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

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

A The sendMessage method is never called, so no messages are sent

B The server property is undefined because the gateway is not instantiated by NestJS

C The @WebSocketServer decorator is missing parentheses

D The emit method requires a callback to confirm delivery

💡 Hint

Check if the method that emits messages is actually used.

The sendMessage method is defined but never called, so no messages are sent. The server property and decorator usage are correct.

🧠 Conceptual

expert

What is the effect of setting 'transports' option in @WebSocketGateway decorator?

Consider this gateway decorator:

@WebSocketGateway({ transports: ['websocket'] })

What does setting transports to ['websocket'] do?

A It causes a syntax error because transports is not a valid option

B It enables both WebSocket and polling transports for clients

C It disables WebSocket transport and uses only polling

D It forces clients to connect only using WebSocket protocol, disabling polling fallback

💡 Hint

Think about how Socket.IO handles transports.

Setting transports to ['websocket'] disables HTTP long-polling fallback and forces clients to connect only via WebSocket.