Angular Subject and BehaviorSubject

What is Subject and BehaviorSubject in Angular?

A Subject is both an Observable and an observer: you can subscribe to it and also push values into it with next(). BehaviorSubject additionally stores the current value and replays it to every new subscriber.

Subject and BehaviorSubject example

@Injectable({ providedIn: 'root' })
export class CartService {
  private itemsSubject = new BehaviorSubject<Job[]>([]);
  items$ = this.itemsSubject.asObservable();

  get items(): Job[] { return this.itemsSubject.value; }

  add(job: Job): void {
    this.itemsSubject.next([...this.items, job]);
  }
}

Key points to remember

• BehaviorSubject for current state; Subject for one-off events.
• Expose it with asObservable() so consumers cannot push values.
• Signals now cover many BehaviorSubject use cases with less ceremony.

Subject variants

Common mistakes with Subject and BehaviorSubject

• Using Subject for state and finding that late subscribers see nothing.
• Reading .value everywhere instead of subscribing, which loses reactivity.