Learn RxJS
Search…
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!
Examples
Example 1: Simple BehaviorSubject
1
// RxJS v6+
2
import { BehaviorSubject } from 'rxjs';
3
4
const subject = new BehaviorSubject(123);
5
6
// two new subscribers will get initial value => output: 123, 123
7
subject.subscribe(console.log);
8
subject.subscribe(console.log);
9
10
// two subscribers will get new value => output: 456, 456
11
subject.next(456);
12
13
// new subscriber will get latest value (456) => output: 456
14
subject.subscribe(console.log);
15
16
// all three subscribers will get new value => output: 789, 789, 789
17
subject.next(789);
18
19
// output: 123, 123, 456, 456, 456, 789, 789, 789
Copied!
Example 2: BehaviorSubject with new subscribers created on mouse clicks
1
// RxJS v6+
2
import { BehaviorSubject, fromEvent, interval, merge } from 'rxjs';
3
import { map, tap, mergeMap } from 'rxjs/operators';
4
5
const setElementText = (elemId, text) =>
6
(document.getElementById(elemId).innerText = text.toString());
7
const addHtmlElement = coords =>
8
(document.body.innerHTML += `
9
<div
10
id=${coords.id}
11
style="
12
position: absolute;
13
height: 30px;
14
width: 30px;
15
text-align: center;
16
top: ${coords.y}px;
17
left: ${coords.x}px;
18
background: silver;
19
border-radius: 80%;"
20
>
21
</div>`);
22
23
const subject = new BehaviorSubject(0);
24
25
const click$ = fromEvent(document, 'click').pipe(
26
map((e: MouseEvent) => ({
27
x: e.clientX,
28
y: e.clientY,
29
id: Math.random()
30
})),
31
tap(addHtmlElement),
32
mergeMap(coords => subject.pipe(tap(v => setElementText(coords.id, v))))
33
);
34
35
const interval$ = interval(1000).pipe(
36
tap(v => subject.next(v)),
37
tap(v => setElementText('intervalValue', v))
38
);
39
40
merge(click$, interval$).subscribe();
Copied!
Related Recipes
Additional Resources
- 📰 - Official docs
Last modified 7mo ago