How to use rxjs to create an object observable from another observable?

Question

I would like to expose an observable in my service, which emits a value each time a BehaviorSubject is assigned a value (and after filtering it from a list). Example implementation:

export class MyService {

   // Given a list of all object
   private readonly allObjects$: Observable<SomeObject[]>;  
   // An id of a SomeObject instance
   private readonly mySubject = new BehaviorSubject<string|undefined>(undefined);

   // Expose a single instance from the list of all objects.
   public readonly myObject$: Observable<SomeObject|undefined>;   

   constructor() {

     
     this.myObject$ = 

        // Pipe in the object id (i.e.: 123)
        this.mySubject.pipe(

        // Add the list of all objects
        withLatestFrom(this.allObjects$),

        // Filter out the object whose id is 123 from the list of objects. This filtered 
        // object should be the value emitted by myObject$.
        switchMap(
            (info: [string, SomeObject[]]) =>
                info[1].filter(t => t.name === info[0])));
   }
}

Usage:

  mySubject.next('123')  
  this.myObject$.subscribe(console.log)  // prints: SomeObject(with id 123)

However the above snippet produces this error (for the withLatestFrom operator):

Argument of type 'OperatorFunction<string | undefined, [string | undefined, 
SomeObject[]]>' is not assignable to parameter of type 'OperatorFunction<string | 
undefined, [string, SomeObject[]]>'.

What am I doing wrong? How do I fix this?

Answer 1

You have defined BehaviorSubject to hold either string or undefined value. So the info array defined within switchMap can have the 0th element as either string or undefined , and hence the type definition needs to specified accordingly. It should be:

    info: [string | undefined, SomeObject[]]

Answer 2

You don't need a higher order observable to subscribe to another observable for this use case, instead of using switchMap(), why don't you use map() or filter()?