BehaviorSubject

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BehaviorSubject

Requires an initial value and emits the current value to new subscribers
💡 If you want the last emitted value(s) on subscription, but do not need to supply a seed value, check out ReplaySubject instead!
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Examples
Example 1: Simple BehaviorSubject
( Stackblitz )
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// RxJS v6+
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import { BehaviorSubject } from 'rxjs';
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​
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const subject = new BehaviorSubject(123);
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​
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// two new subscribers will get initial value => output: 123, 123
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subject.subscribe(console.log);
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subject.subscribe(console.log);
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​
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// two subscribers will get new value => output: 456, 456
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subject.next(456);
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​
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// new subscriber will get latest value (456) => output: 456
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subject.subscribe(console.log);
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​
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// all three subscribers will get new value => output: 789, 789, 789
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subject.next(789);
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​
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// output: 123, 123, 456, 456, 456, 789, 789, 789
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Example 2: BehaviorSubject with new subscribers created on mouse clicks
( Stackblitz )
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// RxJS v6+
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import { BehaviorSubject, fromEvent, interval, merge } from 'rxjs';
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import { map, tap, mergeMap } from 'rxjs/operators';
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​
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const setElementText = (elemId, text) =>
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  (document.getElementById(elemId).innerText = text.toString());
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const addHtmlElement = coords =>
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  (document.body.innerHTML += `
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  <div 
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    id=${coords.id}
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    style="
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      position: absolute;
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      height: 30px;
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      width: 30px;
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      text-align: center;
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      top: ${coords.y}px;
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      left: ${coords.x}px;
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      background: silver;
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      border-radius: 80%;"
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    >
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  </div>`);
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​
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const subject = new BehaviorSubject(0);
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​
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const click$ = fromEvent(document, 'click').pipe(
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  map((e: MouseEvent) => ({
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    x: e.clientX,
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    y: e.clientY,
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    id: Math.random()
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  })),
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  tap(addHtmlElement),
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  mergeMap(coords => subject.pipe(tap(v => setElementText(coords.id, v))))
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);
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​
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const interval$ = interval(1000).pipe(
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  tap(v => subject.next(v)),
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  tap(v => setElementText('intervalValue', v))
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);
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​
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merge(click$, interval$).subscribe();
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Related Recipes
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Additional Resources
​BehaviorSubject​
📰 - Official docs
​BehaviorSubject - In Depth Dev Reference
📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/BehaviorSubject.ts​

AsyncSubject

ReplaySubject

Last modified 6mo ago

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Contents

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

Examples

Related Recipes

Additional Resources