Angular FAQ: Top Questions

Subject and BehaviorSubject are both from RxJS. While Subject emits to subscribers only after subscription, BehaviorSubject holds the last emitted value and emits it immediately to new subscribers.

const sub = new Subject();
sub.next(1); // new subscriber won't receive this

const bsub = new BehaviorSubject(0);
bsub.subscribe(x => console.log(x)); // will receive 0 immediately
        

• BehaviorSubject is ideal for representing current state and broadcasting it.