Event handling in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an event is emitted in NestJS?

Consider a NestJS application where an event is emitted using this.eventEmitter.emit('user.created', user). What is the expected behavior if there is a listener subscribed to the 'user.created' event?

NestJS

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

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

  createUser(user) {
    // user creation logic
    this.eventEmitter.emit('user.created', user);
  }
}

// Listener example
import { Injectable } from '@nestjs/common';
import { OnEvent } from '@nestjs/event-emitter';

@Injectable()
export class UserListener {
  @OnEvent('user.created')
  handleUserCreatedEvent(payload) {
    console.log('User created:', payload);
  }
}

A The eventEmitter throws an error if no listener is found for the event.

B The event is queued and processed only after the application restarts.

C The listener method handleUserCreatedEvent is called with the user object as payload.

D The event is ignored if no listener is registered at the time of emission.

💡 Hint

Think about how event emitters work in NestJS and whether events are synchronous or asynchronous.

In NestJS, when an event is emitted, all listeners subscribed to that event are immediately called with the payload. If no listener exists, the event is simply ignored without error.

📝 Syntax

intermediate

Identify the correct syntax to listen for an event in NestJS

Which of the following code snippets correctly listens to an event named 'order.completed' using NestJS event emitter?

A

@ListenEvent('order.completed')
handleOrderCompleted(payload) {
  console.log(payload);
}

B

@EventListener('order.completed')
handleOrderCompleted(payload) {
  console.log(payload);
}

C

this.eventEmitter.on('order.completed', (payload) => {
  console.log(payload);
});

D

@OnEvent('order.completed')
handleOrderCompleted(payload) {
  console.log(payload);
}

💡 Hint

Recall the decorator provided by NestJS for event listening.

The correct decorator to listen for events in NestJS is @OnEvent(). Other decorators like @EventListener or @ListenEvent do not exist in NestJS.

🔧 Debug

advanced

Why does the event listener not respond to emitted events?

Given the following NestJS code, why does the listener not log anything when createUser is called?

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

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

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

@Injectable()
export class UserListener {
  @OnEvent('user.created')
  handleUserCreated(payload) {
    console.log('User created:', payload);
  }
}

A The event name 'user.created' is misspelled in the emit call.

B The UserListener class is not instantiated because it is not injected anywhere, so its listener method never runs.

C The EventEmitter2 instance is not properly injected into UserListener.

D The listener method handleUserCreated is missing the async keyword.

💡 Hint

Think about how NestJS creates instances of classes and when decorators like @OnEvent are activated.

In NestJS, event listeners decorated with @OnEvent only work if the class is instantiated by the framework. If UserListener is not injected or used anywhere, it is never created, so the listener method never runs.

❓ state_output

advanced

What is the output when multiple listeners handle the same event?

Consider two listeners subscribed to the 'task.finished' event. When the event is emitted with payload { id: 42 }, what will be the console output?

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

@Injectable()
export class ListenerOne {
  @OnEvent('task.finished')
  handleTask(payload) {
    console.log('ListenerOne:', payload.id);
  }
}

@Injectable()
export class ListenerTwo {
  @OnEvent('task.finished')
  handleTask(payload) {
    console.log('ListenerTwo:', payload.id);
  }
}

// Emitting event
this.eventEmitter.emit('task.finished', { id: 42 });

A

ListenerOne: 42
ListenerTwo: 42

B ListenerOne: 42

C ListenerTwo: 42

D No output because multiple listeners cause a conflict.

💡 Hint

Think about how event emitters handle multiple listeners for the same event.

When an event is emitted, all listeners subscribed to that event are called in order. Both ListenerOne and ListenerTwo will log their messages with the payload.

🧠 Conceptual

expert

What error occurs if EventEmitter2 is not provided in a NestJS module?

In a NestJS module, if you try to inject EventEmitter2 without importing EventEmitterModule, what error will you encounter at runtime?

A Nest can't resolve dependencies of the class: No provider for EventEmitter2!

B TypeError: eventEmitter.emit is not a function

C SyntaxError: Unexpected token in event emitter import

D RuntimeError: EventEmitter2 module not found

💡 Hint

Think about how NestJS dependency injection works and what happens if a provider is missing.

If EventEmitterModule is not imported, NestJS does not know how to provide EventEmitter2. This causes a dependency resolution error stating no provider found.