NestJS Issue Fix: How to Inject a Service into a Subscriber

Understanding the Injection Error in NestJS Subscribers

When working with NestJS, a common design pattern involves publishing and subscribing to events. A typical scenario that might lead to an injection issue is when you attempt to use NestJS Dependency Injection (DI) system to inject a service directly into an event subscriber. In NestJS, the DI system relies on the module context to resolve dependencies, and subscribers often fall outside of this context or have a different lifecycle, leading to the injection error you’re encountering.

Therefore, understanding the cause is pivotal: NestJS can’t resolve subscriptions the same way it handles controllers or providers where the DI container has complete awareness of the dependencies.

Solution – Using Modules and Providers Correctly

One way to correct this issue is to ensure that the subscriber is part of a module and recognized by NestJS as a provider. Here’s a step-by-step rundown:

Firstly, define your service class using typical NestJS service syntax with the ‘@Injectable()’ decorator:

@Injectable()
export class MyService {
  // Service methods...
}

Next, ensure this service is included in the appropriate module’s providers array:

@Module({
  providers: [MyService],
})
export class MyModule {}

Then, create a custom provider for the event subscriber if you don’t already have one, marking it with the ‘@Injectable()’ decorator as well. The subscriber class could look something like this:

@Injectable()
export class MyEventSubscriber {
  constructor(private myService: MyService) {}

  handleEvent(payload: any) {
    // Use myService here
  }
}

Now, you can safely inject and utilize the service in your subscriber. Don’t forget to register your subscriber as a provider within the same module:

@Module({
  providers: [MyService, MyEventSubscriber],
})
export class MyModule {}

With these steps completed, NestJS will be able to inject the service into the subscriber successfully as it now recognizes both the service and subscriber within its DI context.

Alternative Solution – Using Instance-based Event Emitters

If you look for a more dynamic or decoupled approach, NestJS also allows instance-based event emitters. This way, you can create an instance of the event emitter in a module where the service is available and inject it into the subscriber dynamically.

This is a bit more complex but offers greater flexibility and encapsulation. You would essentially create an EventEmitter within a service and then inject that service into your subscriber:

@Injectable()
export class EventEmitterService extends EventEmitter {}

@Injectable()
export class MyEventSubscriber {
  constructor(private eventEmitterService: EventEmitterService, private myService: MyService) {
    this.eventEmitterService.on('my-event', (payload) => this.handleEvent(payload));
  }

  private handleEvent(payload: any): void {
    // Use myService here
  }
}

Ensure you add the ‘EventEmitterService’ and ‘MyEventSubscriber’ to the appropriate module’s providers array to effectively inject dependencies.

With either method, you can tackle the dependency injection issue. The key takeaway is that NestJS DI works within the context of its modules, and keeping your services and subscribers within that context or appropriately linked is essential to successful injection.