Upon initialization, Angular's BehaviorSubject is encountered as undefined

Question

Upon initialization, Angular's BehaviorSubject is encountered as undefined

I am in the process of trying to create a fresh BehaviorSubject that is of the type ICar (which I have defined) in a service, with the initial value being either null or undefined. However, when I attempt to change the type to ICar | undefined, I encounter an error in the getCar() function due to a complaint about the return type. Is there a workaround available to resolve this issue?

Here is my code:

export interface ICar{
 id:number,
 name: string,
}
//service:
@Injectable({
    providedIn: 'root'
})
export class CarService {
    private car$ = new BehaviorSubject<ICar>(null); //error

    constructor() {
    }

    setCar(car: ICar) {
        this.car$.next(car);
    }

    getCar(): Observable<ICar> {
        return this.car$;
    }
}

angular typescript rxjs

Answer 1

Answer №1

When dealing with a situation where your car$ can only be null at the beginning and you simply need a behaviorSubject to store the previously loaded value, you can consider using the following approach:

private carSubject = new BehaviorSubject<ICar>(null);
private car$ = carSubject.pipe(filter(car => !!car) car is ICar);

Within the filter operator mentioned above, nullish values are filtered out and it is specified that car is of type ICar.

Additionally, it might be beneficial to rethink the structure of this service. One suggestion could be to prevent external changes to the subject's values (as the current implementation allows passing a reference to the subject outside of the service).

An improved version could look like this:

class CarService implements OnDestroy {
  private readonly car = new BehaviorSubject<ICar | null>(null);

  // By accessing the car$ property, you can retrieve the subject's values as an observable, making it inaccessible for .next calls from external sources
  readonly car$ = car.pipe(filter(car => !!car) car is ICar);

  ngOnDestroy(): void {
    this.car.complete(); // <- Remember to close the subject properly;
  }

  setCar(car: ICar): void {
    this.car.next(car);
  }
}

Answer 2

When dealing with a situation where your car$ can only be null at the beginning and you simply need a behaviorSubject to store the previously loaded value, you can consider using the following approach:

private carSubject = new BehaviorSubject<ICar>(null);
private car$ = carSubject.pipe(filter(car => !!car) car is ICar);

Within the filter operator mentioned above, nullish values are filtered out and it is specified that car is of type ICar.

Additionally, it might be beneficial to rethink the structure of this service. One suggestion could be to prevent external changes to the subject's values (as the current implementation allows passing a reference to the subject outside of the service).

An improved version could look like this:

class CarService implements OnDestroy {
  private readonly car = new BehaviorSubject<ICar | null>(null);

  // By accessing the car$ property, you can retrieve the subject's values as an observable, making it inaccessible for .next calls from external sources
  readonly car$ = car.pipe(filter(car => !!car) car is ICar);

  ngOnDestroy(): void {
    this.car.complete(); // <- Remember to close the subject properly;
  }

  setCar(car: ICar): void {
    this.car.next(car);
  }
}

Answer 3

Answer №2

Consider adjusting your tsoconfig.ts file by including the property

"strictNullChecks": false,

. Proceed with caution as this may not always be the best approach.

https://www.typescriptlang.org/tsconfig#strictNullChecks

Alternatively, a more effective solution would be to indicate that the value/type can be empty, such as ICar | null

OR

Another option is to utilize a Subject instead of a BehaviourSubject and only publish valid ICar to that stream

Answer 4

Consider adjusting your tsoconfig.ts file by including the property

"strictNullChecks": false,

. Proceed with caution as this may not always be the best approach.

https://www.typescriptlang.org/tsconfig#strictNullChecks

Alternatively, a more effective solution would be to indicate that the value/type can be empty, such as ICar | null

OR

Another option is to utilize a Subject instead of a BehaviourSubject and only publish valid ICar to that stream

Upon initialization, Angular's BehaviorSubject is encountered as undefined

Answer №1

Answer №2

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