Why should you care?

Multiple components need the same current user or theme; Subject bridges imperative code and Observable templates.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

@Injectable({ providedIn: 'root' })
export class AuthStore {
  private userSubject = new BehaviorSubject<User | null>(null);
  user$ = this.userSubject.asObservable();

  setUser(user: User | null): void {
    this.userSubject.next(user);
  }

  snapshot(): User | null {
    return this.userSubject.getValue();
  }
}

What happened?

• BehaviorSubject starts with null.
• asObservable() hides next() from consumers — they only subscribe.
• getValue() reads current user synchronously for guards.

Practice next

1. Create BehaviorSubject for selected account id.
2. Expose read-only user$ without exposing subject.
3. Call setUser after login API succeeds.
4. Replace BehaviorSubject with toObservable(signal) in new code.
5. Use ReplaySubject(1) for last API error message.

Remember

Subject = Observable you write to. BehaviorSubject stores latest value. Hide subject; expose Observable API.

Session broadcast

Logout in header must clear dashboard and sidebar instantly.

Outcome: AuthStore.next(null) updates all subscribers together.