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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!

Why use BehaviorSubject?

This specialized subject is ideal when you want to maintain and provide a "current value" to subscribers. Think of it as a scoreboard in a basketball game. Even if you join watching in the middle of the game, you'll still see the current score. Similarly, when a new observer subscribes to a BehaviorSubject, it immediately receives the current value (or the last value that was emitted).
It's important to remember that a BehaviorSubject requires an initial value upon instantiation. This is where it differs from a regular Subject which doesn't have an initial value. Picture a newly installed scoreboard – with BehaviorSubject, you set a starting score, say 0-0. With a regular Subject, the board remains blank until a point is scored. Subscribers (early or late) of a normal Subject will not receive emissions until the Subject emits a value.
Contrasting with ReplaySubject, while both provide historical values, ReplaySubject can relay multiple previous values, not just the last one. If the basketball scoreboard could show the last five scores in the match sequence, that'd be akin to ReplaySubject. ReplaySubject also does not receive an initial seed value.
In conclusion, if you need to ensure subscribers always get the latest value upon subscription, or you have an initial seed value, BehaviorSubject is your pick. If you need more historical emissions, consider ReplaySubject. And if you don't need any history at all, a simple Subject might be what you're looking for.
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Examples

Example 1: Simple BehaviorSubject
// 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
// 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