Event handling in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the EventEmitter2 class from NestJS.

NestJS

import { [1] } from '@nestjs/event-emitter';

Drag options to blanks, or click blank then click option'

AEventDispatcher

BEventHandler

CEventListener

DEventEmitter2

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class names like EventHandler or EventListener.

Forgetting to import from '@nestjs/event-emitter'.

The EventEmitter2 class is imported from @nestjs/event-emitter to handle events in NestJS.

2fill in blank

medium

Complete the code to inject the EventEmitter2 into the service constructor.

NestJS

constructor(private readonly [1]: EventEmitter2) {}

Drag options to blanks, or click blank then click option'

AeventEmitter

BeventHandler

CeventListener

DeventDispatcher

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect property names that do not match the class.

Forgetting the private readonly keywords.

The injected property should be named eventEmitter to clearly represent the EventEmitter2 instance.

3fill in blank

hard

Fix the error in emitting an event with a payload.

NestJS

this.eventEmitter.[1]('user.created', { id: 1, name: 'Alice' });

Drag options to blanks, or click blank then click option'

AemitEvent

Bdispatch

Cemit

Dtrigger

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like emitEvent or dispatch.

Confusing with other event libraries.

The correct method to emit an event in NestJS EventEmitter2 is emit.

4fill in blank

hard

Fill both blanks to create an event listener method that listens to 'order.placed' events.

NestJS

@[1]('order.placed')
handleOrderPlaced([2]: any) {
  console.log('Order received:', [2]);
}

Drag options to blanks, or click blank then click option'

AOnEvent

BEventHandler

Corder

Dpayload

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect decorator names like @EventHandler.

Using unclear parameter names.

The @OnEvent decorator listens to events, and the method parameter is commonly named payload to hold event data.

5fill in blank

hard

Fill all three blanks to emit an event and handle it properly in NestJS.

NestJS

this.[1].[2]('payment.completed', { amount: 100 });

@[3]('payment.completed')
handlePaymentCompleted(payload: any) {
  console.log('Payment done:', payload.amount);
}

Drag options to blanks, or click blank then click option'

AeventEmitter

Bemit

COnEvent

Ddispatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names like 'dispatch'.

Using wrong decorator names.

Use eventEmitter.emit to send events and @OnEvent decorator to listen to them.

Practice

1. What is the main purpose of event handling in a NestJS application?

easy

A. To allow different parts of the app to react to actions without being tightly connected

B. To directly call methods from other classes

C. To replace all HTTP requests with events

D. To store data permanently in the database

Solution

1. Step 1: Understand event handling concept

   Event handling lets parts of an app respond to actions independently, avoiding tight coupling.

2. Step 2: Compare options

   Only To allow different parts of the app to react to actions without being tightly connected describes this purpose correctly; others describe unrelated concepts.

3. Final Answer:

   To allow different parts of the app to react to actions without being tightly connected -> Option A

4. Quick Check:

   Event handling = loose coupling reaction [OK]

Hint: Event handling means reacting without tight connections [OK]

Common Mistakes:

• Thinking event handling replaces HTTP requests
• Confusing event handling with direct method calls
• Assuming event handling stores data

2. Which decorator is used to mark a class as an event handler in NestJS?

easy

A. @HandleEvent

B. @EventListener

C. @EventsHandler

D. @EventSubscriber

Solution

1. Step 1: Recall NestJS event handler syntax

   The correct decorator to mark event handler classes is @EventsHandler.

2. Step 2: Verify options

   Options B, C, and D are not valid NestJS decorators for event handlers.

3. Final Answer:

   @EventsHandler -> Option C

4. Quick Check:

   @EventsHandler marks event handlers [OK]

Hint: Use @EventsHandler to mark event handler classes [OK]

Common Mistakes:

• Using @EventListener which is not a NestJS decorator
• Confusing with @HandleEvent which does not exist
• Using @EventSubscriber which is from other frameworks

3. Given this event handler code, what will be logged when eventBus.publish(new UserCreatedEvent('Alice')) is called?

import { EventsHandler, IEventHandler } from '@nestjs/cqrs';

export class UserCreatedEvent {
  constructor(public readonly username: string) {}
}

@EventsHandler(UserCreatedEvent)
export class UserCreatedHandler implements IEventHandler<UserCreatedEvent> {
  handle(event: UserCreatedEvent) {
    console.log(`User created: ${event.username}`);
  }
}

medium

A. User created: undefined

B. No output because eventBus.publish does nothing

C. Error: handle method missing

D. User created: Alice

Solution

1. Step 1: Understand event publishing and handling

   Publishing UserCreatedEvent triggers UserCreatedHandler which logs the username.

2. Step 2: Check the handle method output

   The handle method logs "User created: " plus the event's username, which is "Alice".

3. Final Answer:

   User created: Alice -> Option D

4. Quick Check:

   Event publish triggers handler log [OK]

Hint: Publishing event triggers handle() logging username [OK]

Common Mistakes:

• Assuming username is undefined
• Thinking handle method is missing
• Believing eventBus.publish does nothing

4. What is wrong with this event handler code?

import { EventsHandler, IEventHandler } from '@nestjs/cqrs';

export class OrderPlacedEvent {
  constructor(public readonly orderId: number) {}
}

@EventsHandler(OrderPlacedEvent)
export class OrderPlacedHandler implements IEventHandler<OrderPlacedEvent> {
  handle() {
    console.log('Order placed:', orderId);
  }
}

medium

A. The handle method is missing the event parameter

B. The @EventsHandler decorator is used incorrectly

C. The event class should not have a constructor

D. The console.log syntax is invalid

Solution

1. Step 1: Check handle method signature

   The handle method must accept the event parameter to access event data.

2. Step 2: Identify usage of event data

   Inside handle, orderId is used but not defined because event parameter is missing.

3. Final Answer:

   The handle method is missing the event parameter -> Option A

4. Quick Check:

   handle(event) needs event parameter [OK]

Hint: handle() must receive event parameter to access data [OK]

Common Mistakes:

• Omitting event parameter in handle method
• Misusing @EventsHandler decorator
• Thinking constructor is not needed in event class

5. You want to handle two different events, UserCreatedEvent and UserDeletedEvent, in one handler class. Which approach correctly implements this in NestJS?

hard

A. Use @EventsHandler without parameters and check event type inside handle method

B. Use @EventsHandler(UserCreatedEvent, UserDeletedEvent) on one class and implement handle(event) with type checks

C. Use @EventsHandler on one class but implement two separate handle methods named handleUserCreated and handleUserDeleted

D. Create two separate handler classes each with @EventsHandler for one event

Solution

1. Step 1: Understand multiple event handling in one class

   In NestJS, you can pass multiple event classes to @EventsHandler decorator on one class.

2. Step 2: Implement single handle method with event type checks

   The handle method receives any event instance; inside it, you check the event type to handle accordingly.

3. Final Answer:

   Use @EventsHandler(UserCreatedEvent, UserDeletedEvent) on one class and implement handle(event) with type checks -> Option B

4. Quick Check:

   Multiple events in one handler use @EventsHandler(...events) [OK]

Hint: Use @EventsHandler with multiple events and one handle method [OK]

Common Mistakes:

• Trying to define multiple handle methods in one class
• Creating separate classes instead of one combined handler
• Using @EventsHandler without parameters