const observable = new rxjs.BehaviorSubject(0);

observable.subscribe(v => console.log(v));

rxjs
  .of(1)
  .pipe(rxjs.operators.delay(500))
  .subscribe(v => observable.next(v));
  
observable.next(2);

<script src=".5.1/rxjs.umd.js"></script>

const observable = new rxjs.BehaviorSubject(0);

observable.subscribe(v => console.log(v));

rxjs
  .of(1)
  .pipe(rxjs.operators.delay(500))
  .subscribe(v => observable.next(v));
  
observable.next(2);

<script src="https://cdnjs.cloudflare./ajax/libs/rxjs/6.5.1/rxjs.umd.js"></script>

As you can see, the observable above emits 3 values in order : 0, 2, 1.

Would it be possible to cancel (or ignore) the value "1" when the value "2" is emitted ? (Without closing the subscription)

• javascript
• rxjs
• observable

• There are better ways to emit numbers one after another, rxjs timer for example. in any case, if you have an observable with subscription and want that new value that emited will cancel the last subscription and switch to a new one read about switchMap. But i'm not sure if it's possible in this code that you added above – Lagistos Commented Apr 25, 2019 at 14:26
• @Lagistos this is a minimal reproducible example. I used numbers to be quick. In reality, the nexts are called on mouse events. I would like to cancel the delayed mouseenter on mouseleave, because otherwise, the leave emits, then the enter does too. – user4676340 Commented Apr 25, 2019 at 14:28
• Based on what condition you want to ignore the last value? You could use ignoreElements() before delay if you never want to receive the value from of. Or you can also use EMPTY as your source Observable. – martin Commented Apr 25, 2019 at 14:31
• There's no condition, I would like to cancel a delayed emition if a new emition es up. I know that you can use SwitchMap for that, but I don't know how to use it for a single observable. – user4676340 Commented Apr 25, 2019 at 14:36
• 1 use debounceTime(500) instead of delay(500). @trichetriche – n00dl3 Commented Apr 25, 2019 at 16:20

3 Answers 3

Seems like you need to switchMap from your source and apply a delay inside of it.

switchMap(value =>
 of(value).pipe(delay(50))
)

An illustration and a playground for switchMap with a delay:

And heres a snippet:

const {Subject, of} = rxjs;
const {switchMap, delay} = rxjs.operators;

const subject = new Subject(0);

subject
  .pipe(
     switchMap(value =>
       // switchMap to a delayed value
       of(value).pipe(delay(500))
     )
  )
  .subscribe(v => console.log(v));

// immediately emit 0
subject.next(0);

// emit 1 in 1 sec
setTimeout(()=>{
  subject.next(1);
}, 1000)

// emit 2 in 1.2 sec
setTimeout(()=>{
  subject.next(2);
}, 1200)

<script src="https://cdnjs.cloudflare./ajax/libs/rxjs/6.5.1/rxjs.umd.js"></script>

Heres an example with mousehover

const {fromEvent, merge, of, EMPTY} = rxjs;
const {switchMap, delay, mapTo} = rxjs.operators;

const button = document.getElementById('pane');
const mouseOver$ = fromEvent(button, 'mouseover').pipe(
  mapTo(true)
);

const mouseOut$ = fromEvent(button, 'mouseout').pipe(
  mapTo(false)
);

merge(mouseOver$, mouseOut$)
  .pipe(
     switchMap(value => {
       if (!value) { return EMPTY; }
       return of('mouse is over').pipe(delay(500))
     })
  )
  .subscribe(v => console.log(v));

<script src="https://cdnjs.cloudflare./ajax/libs/rxjs/6.5.1/rxjs.umd.js"></script>

<style>
#pane{
  margin: 1rem;
  display: inline-block;
  width: 5rem;
  height: 5rem;
  background: rebeccapurple;
}</style>

<div id="pane"><div>

Hope this helps

The operator you are looking for is debounceTime :

debounceTime

Emits a value from the source Observable only after a particular time span has passed without another source emission.

source

rxjs.interval(100)
  .pipe(
    rxjs.operators.take(10),
    rxjs.operators.debounceTime(500)
  )
  .subscribe((v)=>{
    console.log(v);
  });

<script src="https://cdnjs.cloudflare./ajax/libs/rxjs/6.5.1/rxjs.umd.js"></script>

So for mouse enter and leave you are looking for debounceTime for example:

const observable = new BehaviorSubject(0);
observable
  .pipe(debounceTime(500))
 .subscribe(console.log);

observable.next(1),
observable.next(2);
setTimeout(() => observable.next(3) , 1000)

In this example that will print 2 and after one seconed 3. After each emitited value the observable wait for 500 ms and if there are no new value it will print in the subscribe else it will cancel the last one and start this proccess again, hope this will solve your problem