How to Use Event Emitter in NestJS: Simple Guide

In NestJS, use the built-in EventEmitter2 package to emit and listen to events by injecting EventEmitter2 service. Emit events with emit() and listen using @OnEvent() decorator in your services or controllers.

Syntax

To use event emitter in NestJS, first import and inject EventEmitter2 from @nestjs/event-emitter. Use emit(eventName, payload) to send events. Use @OnEvent(eventName) decorator on methods to listen for events.

• EventEmitter2: The service to emit events.
• emit(eventName, payload): Sends an event with a name and optional data.
• @OnEvent(eventName): Decorator to mark event listener methods.

typescript

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

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

  triggerEvent() {
    this.eventEmitter.emit('my.event', { message: 'Hello' });
  }

  @OnEvent('my.event')
  handleMyEvent(payload: any) {
    console.log('Event received:', payload);
  }
}

Example

This example shows a service emitting an event and another service listening to it. When triggerEvent() is called, the listener logs the event payload.

typescript

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

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

  triggerEvent() {
    this.eventEmitter.emit('user.created', { userId: 123, name: 'Alice' });
  }
}

@Injectable()
export class ListenerService {
  @OnEvent('user.created')
  handleUserCreatedEvent(payload: { userId: number; name: string }) {
    console.log(`User created with ID: ${payload.userId} and name: ${payload.name}`);
  }
}

// Usage example (e.g., in a controller or main app):
// const emitter = new EmitterService(eventEmitterInstance);
// emitter.triggerEvent();

Output

User created with ID: 123 and name: Alice

Common Pitfalls

Common mistakes when using event emitter in NestJS include:

• Not importing EventEmitterModule in your module, so the event emitter service is not available.
• Forgetting to decorate listener methods with @OnEvent(), so events are not caught.
• Using string event names inconsistently, causing listeners to miss events.
• Not handling asynchronous listeners properly if needed.

typescript

/* Wrong: Missing EventEmitterModule import */
// @Module({
//   providers: [EmitterService, ListenerService],
// })
// export class AppModule {}

/* Right: Import EventEmitterModule */
import { Module } from '@nestjs/common';
import { EventEmitterModule } from '@nestjs/event-emitter';

@Module({
  imports: [EventEmitterModule.forRoot()],
  providers: [EmitterService, ListenerService],
})
export class AppModule {}

Quick Reference

Summary tips for using EventEmitter in NestJS:

• Always import EventEmitterModule.forRoot() in your root module.
• Inject EventEmitter2 to emit events.
• Use @OnEvent('event.name') to listen to events.
• Keep event names consistent and descriptive.
• Listeners can be async if needed.

Key Takeaways

Import EventEmitterModule.forRoot() in your module to enable event emitting.

Inject EventEmitter2 service to emit events with emit(eventName, payload).

Use @OnEvent(eventName) decorator on methods to listen for events.

Keep event names consistent to ensure listeners receive events.

Listeners can be asynchronous to handle async operations.