39. BehaviorSubject: current state

BehaviorSubject — a Subject that remembers the current value

BehaviorSubject is a Subject that remembers the last value. So:

• It needs an initial value when created.
• Every new subscriber instantly gets the current value (a snapshot).
• You can read the value synchronously via .getValue().

const user$ = new BehaviorSubject('Guest');
user$.subscribe(v => console.log('A:', v));  // → A: Guest (instantly)
user$.next('Admin');                         // → A: Admin
user$.subscribe(v => console.log('B:', v));  // → B: Admin (instantly, the latest)
user$.getValue();                             // → 'Admin' (synchronously)

Where to use it

• The current user / theme / language — state that always has a value.
• Simple state storage in an Angular service.
• Any case where a "late" subscriber must immediately learn the current value.

const { BehaviorSubject } = Rx;

const user$ = new BehaviorSubject('Guest');

user$.subscribe(value => console.log('Sub A: ' + value));

user$.next('Admin');

user$.subscribe(value => console.log('Sub B: ' + value));

console.log('Current: ' + user$.getValue());