BehaviorSubject

Requires an initial value and emits the current value to new subscribers


:bulb: If you want the last emitted value(s) on subscription, but do not need to supply a seed value, check out ReplaySubject instead!


Examples

Example 1: Simple BehaviorSubject

( Stackblitz )

// RxJS v6+
import { BehaviorSubject } from 'rxjs';

const subject = new BehaviorSubject(123);

// two new subscribers will get initial value => output: 123, 123
subject.subscribe(console.log);
subject.subscribe(console.log);

// two subscribers will get new value => output: 456, 456
subject.next(456);

// new subscriber will get latest value (456) => output: 456
subject.subscribe(console.log);

// all three subscribers will get new value => output: 789, 789, 789
subject.next(789);

// output: 123, 123, 456, 456, 456, 789, 789, 789
Example 2: BehaviorSubject with new subscribers created on mouse clicks

( Stackblitz )

// RxJS v6+
import { BehaviorSubject, fromEvent, interval, merge } from 'rxjs';
import { map, tap, mergeMap } from 'rxjs/operators';

const setElementText = (elemId, text) =>
  (document.getElementById(elemId).innerText = text.toString());
const addHtmlElement = coords =>
  (document.body.innerHTML += `
  <div 
    id=${coords.id}
    style="
      position: absolute;
      height: 30px;
      width: 30px;
      text-align: center;
      top: ${coords.y}px;
      left: ${coords.x}px;
      background: silver;
      border-radius: 80%;"
    >
  </div>`);

const subject = new BehaviorSubject(0);

const click$ = fromEvent(document, 'click').pipe(
  map((e: MouseEvent) => ({
    x: e.clientX,
    y: e.clientY,
    id: Math.random()
  })),
  tap(addHtmlElement),
  mergeMap(coords => subject.pipe(tap(v => setElementText(coords.id, v))))
);

const interval$ = interval(1000).pipe(
  tap(v => subject.next(v)),
  tap(v => setElementText('intervalValue', v))
);

merge(click$, interval$).subscribe();

Additional Resources


:file_folder: Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/BehaviorSubject.ts

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