Filter all 'null' values from an Observable<T>

Add a filter operator to your observable chain. You can filter nulls explicitly or just check that your user is truthy - you will have to make that call depending on your needs.

Filtering out null users only:

public currentUser = this.currentUserSubject
                         .asObservable()
                         .filter(user => user !== null)
                         .distinctUntilChanged(); 

Another way to check the value exists:

public currentUser = this.currentUserSubject
                         .asObservable()
                         .filter<User>(Boolean)
                         .distinctUntilChanged(); 

with rxjs@6 and typescript the recommended (readable/maintainable) way is to define the following type guard:

export function isNonNull<T>(value: T): value is NonNullable<T> {
  return value != null;
}

and augment a subject with pipe and filter:

subject.pipe(filter(isNonNull))