Event handling in NestJS - Mini Project: Build & Apply

Event Handling in NestJS

📖 Scenario: You are building a simple notification system in a NestJS application. When a new user registers, the system should emit an event and handle it to send a welcome message.

🎯 Goal: Create a NestJS event emitter and listener to handle user registration events and send a welcome notification.

📋 What You'll Learn

Create an event payload interface for user data

Set up an event emitter service to emit user registration events

Create an event listener to handle the registration event

Log a welcome message when the event is handled

💡 Why This Matters

🌍 Real World

Event handling is common in backend applications to decouple actions, such as sending notifications after user actions.

💼 Career

Understanding event-driven architecture and NestJS event handling is valuable for backend developers working with scalable and maintainable applications.

1

Create User Registration Event Payload

Create an interface called UserRegisteredEvent with two string properties: username and email.

NestJS

# Your code here

Need a hint?

Interfaces in TypeScript define the shape of an object. Use interface UserRegisteredEvent { username: string; email: string; }.

2

Set Up Event Emitter Service

Create a class called UserEventsService that injects EventEmitter2 from @nestjs/event-emitter. Add a method emitUserRegistered that takes a UserRegisteredEvent parameter and emits an event named user.registered with the event data.

NestJS

export interface UserRegisteredEvent {
  username: string;
  email: string;
}

// Your code here

Need a hint?

Use dependency injection to get EventEmitter2. Then call emit with the event name and data.

3

Create Event Listener for User Registration

Create a class called UserEventsListener with a method handleUserRegistered that listens to the user.registered event using the @OnEvent('user.registered') decorator. The method should accept a UserRegisteredEvent parameter.

NestJS

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

export interface UserRegisteredEvent {
  username: string;
  email: string;
}

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

  emitUserRegistered(event: UserRegisteredEvent) {
    this.eventEmitter.emit('user.registered', event);
  }
}

// Your code here

Need a hint?

Use the @OnEvent decorator to listen to events. The method receives the event data as a parameter.

4

Log Welcome Message in Event Listener

Inside the handleUserRegistered method of UserEventsListener, add a console.log statement that outputs: Welcome, {username}! Your email is {email}. using the event data.

NestJS

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

export interface UserRegisteredEvent {
  username: string;
  email: string;
}

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

  emitUserRegistered(event: UserRegisteredEvent) {
    this.eventEmitter.emit('user.registered', event);
  }
}

@Injectable()
export class UserEventsListener {
  @OnEvent('user.registered')
  handleUserRegistered(event: UserRegisteredEvent) {
    // Add console.log here
  }
}

Need a hint?

Use a template string with backticks to include variables inside the message.