Event patterns (event-based) in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an event handler using the correct decorator.

NestJS

import { Controller } from '@nestjs/common';
import { EventPattern } from '@nestjs/microservices';

@Controller()
export class AppController {
  @[1]('order_created')
  handleOrderCreated(data: any) {
    console.log('Order created event received:', data);
  }
}

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

ASubscribeMessage

BEventPattern

CMessagePattern

DOnEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using @MessagePattern instead of @EventPattern

Using @SubscribeMessage which is for WebSocket events

Using @OnEvent which is for internal event emitter

The @EventPattern decorator is used in NestJS to listen for specific events in event-based communication.

2fill in blank

medium

Complete the code to emit an event named 'user_registered' with user data.

NestJS

import { ClientProxy } from '@nestjs/microservices';

export class UserService {
  constructor(private client: ClientProxy) {}

  registerUser(user: any) {
    // user registration logic
    this.client.[1]('user_registered', user);
  }
}

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

Asend

Bpublish

Cdispatch

Demit

Attempts:

3 left

💡 Hint

Common Mistakes

Using send which expects a response

Using publish which is not a ClientProxy method

Using dispatch which does not exist

The emit method on ClientProxy is used to send events to the event bus in NestJS.

3fill in blank

hard

Fix the error in the event handler method signature to properly receive event data.

NestJS

import { Controller } from '@nestjs/common';
import { EventPattern } from '@nestjs/microservices';

@Controller()
export class PaymentController {
  @EventPattern('payment_processed')
  handlePayment([1]) {
    console.log('Payment processed:', data);
  }
}

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

Adata

Bevent

Cpayload

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name different from what is used inside the method

Not defining any parameter to receive event data

The event handler method should receive the event data as a parameter, commonly named data.

4fill in blank

hard

Fill both blanks to create a service that listens to 'inventory_updated' events and logs the update.

NestJS

import { Injectable } from '@nestjs/common';
import { [1] } from '@nestjs/microservices';

@Injectable()
export class InventoryService {
  @[2]('inventory_updated')
  handleInventoryUpdate(update: any) {
    console.log('Inventory updated:', update);
  }
}

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

AEventPattern

BMessagePattern

CSubscribeMessage

DOnEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Importing and using different decorators

Using MessagePattern which is for request-response

Both the import and the decorator should be EventPattern to listen to event-based messages.

5fill in blank

hard

Fill all three blanks to create a client service that emits 'notification_sent' events with a message payload.

NestJS

import { Injectable } from '@nestjs/common';
import { [1] } from '@nestjs/microservices';

@Injectable()
export class NotificationService {
  constructor(private client: [2]) {}

  sendNotification(message: string) {
    this.client.[3]('notification_sent', { message });
  }
}

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

AClientProxy

Cemit

Dsend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'send' instead of 'emit' for events

Mismatching injected client type and import

The ClientProxy is imported and injected to emit events using the emit method.

Practice

1. What is the main purpose of using @EventPattern in a NestJS application?

easy

A. To create database entities

B. To define HTTP routes for REST APIs

C. To listen for specific events and handle their data

D. To style components with CSS

Solution

1. Step 1: Understand the role of @EventPattern

   This decorator is used to mark a method that listens for a specific event in an event-based system.

2. Step 2: Compare with other options

   Options B, C, and D relate to other parts of NestJS or frontend development, not event listening.

3. Final Answer:

   To listen for specific events and handle their data -> Option C

4. Quick Check:

   @EventPattern listens to events [OK]

Hint: Remember: @EventPattern listens to events, not routes or styles [OK]

Common Mistakes:

• Confusing @EventPattern with @Controller for routes
• Thinking @EventPattern creates database models
• Mixing event listening with styling tasks

2. Which of the following is the correct way to receive event data inside an event handler method in NestJS?

easy

A. Use @Payload() decorator on a method parameter

B. Use @Body() decorator on a method parameter

C. Use @Query() decorator on a method parameter

D. Use @Param() decorator on a method parameter

Solution

1. Step 1: Identify the decorator for event data

   In NestJS event handlers, @Payload() extracts the data sent with the event.

2. Step 2: Differentiate from HTTP decorators

   @Body(), @Query(), and @Param() are used in HTTP controllers, not event handlers.

3. Final Answer:

   Use @Payload() decorator on a method parameter -> Option A

4. Quick Check:

   @Payload() gets event data [OK]

Hint: Event data comes from @Payload(), not HTTP decorators [OK]

Common Mistakes:

• Using @Body() in event handlers instead of @Payload()
• Confusing HTTP request decorators with event decorators
• Omitting the decorator and expecting automatic data injection

3. Given the following NestJS event handler code, what will be logged when an event named 'order_created' with payload { id: 42 } is emitted?

@EventPattern('order_created')
handleOrder(@Payload() data: any) {
  console.log(`Order ID: ${data.id}`);
}

medium

A. No output because event name is wrong

B. Order ID: undefined

C. SyntaxError due to missing return

D. Order ID: 42

Solution

1. Step 1: Understand event matching

   The handler listens for 'order_created' events, so it will trigger when such an event is emitted.

2. Step 2: Check payload usage

   The payload has an object with id: 42, so data.id is 42 and will be logged.

3. Final Answer:

   Order ID: 42 -> Option D

4. Quick Check:

   Payload id 42 logs correctly [OK]

Hint: Match event name exactly and use @Payload() to access data [OK]

Common Mistakes:

• Assuming event name is case-insensitive
• Forgetting to use @Payload() and getting undefined
• Expecting a return value to log output

4. Identify the error in this NestJS event handler code:

@EventPattern('user_registered')
handleUser(@Payload data: any) {
  console.log(data.username);
}

medium

A. Missing parentheses in @Payload decorator

B. Incorrect event name format

C. Method name must be 'onUserRegistered'

D. Payload parameter must be last in method

Solution

1. Step 1: Check decorator syntax

   The @Payload decorator requires parentheses even if no arguments are passed, so it should be @Payload().

2. Step 2: Verify other parts

   Event name format is correct, method name can be anything, and parameter order is flexible.

3. Final Answer:

   Missing parentheses in @Payload decorator -> Option A

4. Quick Check:

   @Payload() needs parentheses [OK]

Hint: Always write @Payload() with parentheses, even if empty [OK]

Common Mistakes:

• Omitting parentheses in decorators
• Thinking method names must follow event names
• Assuming parameter order matters for decorators

5. You want to create a NestJS service that listens to two different events: 'payment_success' and 'payment_failed'. Which of the following is the best way to implement this using event patterns?

hard

A. Use @Controller with two routes named after events

B. Create two methods, each decorated with @EventPattern for each event name

C. Create one method with @EventPattern('payment') and check event type inside

D. Use @Payload() decorator twice in one method for both events

Solution

1. Step 1: Understand event pattern usage

   Each event should have its own handler method decorated with @EventPattern specifying the event name.

2. Step 2: Evaluate other options

   Create one method with @EventPattern('payment') and check event type inside is less clear and not idiomatic; Use @Controller with two routes named after events is for HTTP routes, not events; Use @Payload() decorator twice in one method for both events is invalid syntax.

3. Final Answer:

   Create two methods, each decorated with @EventPattern for each event name -> Option B

4. Quick Check:

   Separate methods for separate events [OK]

Hint: Use one @EventPattern per event with separate methods [OK]

Common Mistakes:

• Trying to handle multiple events in one method without checking
• Confusing event handlers with HTTP controllers
• Using multiple @Payload decorators in one method