Event patterns (event-based) in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when an event is emitted?

Consider a NestJS event emitter setup where a listener logs a message when an event named 'user.created' is emitted. What will be logged when the event is emitted with payload { id: 42 }?

NestJS

import { Injectable } from '@nestjs/common';
import { EventEmitter2, OnEvent } from '@nestjs/event-emitter';

@Injectable()
export class UserService {
  constructor(private eventEmitter: EventEmitter2) {}

  createUser() {
    this.eventEmitter.emit('user.created', { id: 42 });
  }
}

@Injectable()
export class UserListener {
  @OnEvent('user.created')
  handleUserCreatedEvent(payload: { id: number }) {
    console.log(`User created with id: ${payload.id}`);
  }
}

A No output, event not handled

B User created with id: undefined

C User created with id: 42

D Error: EventEmitter2 is not defined

💡 Hint

Think about how the @OnEvent decorator works and what payload is passed to the listener.

The @OnEvent decorator listens for the 'user.created' event. When the event is emitted with payload { id: 42 }, the listener logs the message with the id value.

📝 Syntax

intermediate

Which option correctly defines an event listener in NestJS?

You want to listen to an event named 'order.completed' in a NestJS service. Which code snippet correctly sets up the listener?

A

@OnEvent('order.completed')
handleOrderCompleted(payload: any) {
  console.log('Order completed:', payload);
}

B

@EventListener('order.completed')
handleOrderCompleted(payload: any) {
  console.log('Order completed:', payload);
}

C

@Subscribe('order.completed')
handleOrderCompleted(payload: any) {
  console.log('Order completed:', payload);
}

D

@ListenEvent('order.completed')
handleOrderCompleted(payload: any) {
  console.log('Order completed:', payload);
}

💡 Hint

Check the official NestJS event emitter decorator name.

The correct decorator to listen to events in NestJS is @OnEvent. Other decorators are invalid and will cause errors.

🔧 Debug

advanced

Why does the event listener not respond?

Given the following NestJS code, why does the listener not log anything when the event is emitted?

NestJS

import { Injectable } from '@nestjs/common';
import { EventEmitter2, OnEvent } from '@nestjs/event-emitter';

@Injectable()
export class NotificationService {
  constructor(private eventEmitter: EventEmitter2) {}

  sendNotification() {
    this.eventEmitter.emit('notification.sent', { message: 'Hello' });
  }
}

@Injectable()
export class NotificationListener {
  @OnEvent('notification.send')
  handleNotification(payload: { message: string }) {
    console.log('Notification:', payload.message);
  }
}

A The event name in the listener ('notification.send') does not match the emitted event name ('notification.sent').

B The listener method is missing the async keyword.

C EventEmitter2 is not injected properly in NotificationListener.

D The payload type is incorrect and causes a runtime error.

💡 Hint

Check the event names carefully for exact matches.

Event names must match exactly. Here, the listener listens for 'notification.send' but the event emitted is 'notification.sent', so the listener never triggers.

❓ state_output

advanced

What is the final state after emitting multiple events?

A NestJS service emits two events in sequence. A listener updates a count each time it receives the event. What is the final count after both events?

NestJS

import { Injectable } from '@nestjs/common';
import { EventEmitter2, OnEvent } from '@nestjs/event-emitter';

@Injectable()
export class CounterService {
  private count = 0;

  constructor(private eventEmitter: EventEmitter2) {}

  increment() {
    this.eventEmitter.emit('counter.increment');
  }

  getCount() {
    return this.count;
  }
}

@Injectable()
export class CounterListener {
  private count = 0;

  @OnEvent('counter.increment')
  handleIncrement() {
    this.count += 1;
  }

  getCount() {
    return this.count;
  }
}

// In application code:
// counterService.increment();
// counterService.increment();
// What is counterListener.getCount()?

A 0

B Error: count is private and inaccessible

C 1

D 2

💡 Hint

Events decouple the emitter and listener. The listener updates its own state independently each time the event is emitted.

The CounterService emits the 'counter.increment' event twice. The CounterListener receives both events via the @OnEvent decorator and increments its private count each time. Therefore, counterListener.getCount() returns 2. Note that the service has its own unused count which remains 0.

🧠 Conceptual

expert

Which statement best describes event propagation in NestJS event emitter?

In NestJS using EventEmitter2, what happens when multiple listeners are registered for the same event and one listener throws an error during event handling?

A Listeners are called asynchronously; errors in one listener do not affect others, all listeners run regardless.

B All listeners are called in order; if one throws an error, the error is propagated and remaining listeners are not called.

C Listeners are called synchronously; if one throws an error, it is caught internally and logged, allowing others to run.

D Only the first listener is called; if it throws an error, event propagation stops immediately.

💡 Hint

Think about synchronous event emission and error handling in EventEmitter2 by default.

By default, EventEmitter2 calls listeners synchronously in order. If a listener throws an error, the error propagates and stops further listeners from running unless caught.