How to remove specific element from Observable<Array<any>>

Question

There is an Observable of the array of places:

places: Observable<Array<any>>;

In template it used with the async pipe:

<tr *ngFor="let place of places | async">
  ...
</tr>

After some user actions I need to remove the place with specific id from this array. I have something like this in my code, but it doesn't work:

deletePlace(placeId: number): void {
  this.apiService.deletePlace(placeId)
  .subscribe(
    (res: any) => {
      this.places
        .flatMap((places) => places)
        .filter((place) => place.id != placeId);
    }, 
    (err: any) => console.log(err)
  );    
}  

Can you help me with this?

Answer 1

You can't do it this way since you can't "update" an observable (ie it doesn't keep states) but you can react to an event through it.

For your use case, I would leverage the scan operator and merge two streams into a single one:

• one for the initial loading
• another one for the delete event.

Here is a sample:

let obs = this.http.get('/data').map(res => res.json());

this.deleteSubject = new Subject();

this.mergedObs = obs.merge(this.deleteSubject)
.startWith([])
.scan((acc, val) => {
  if (val.op && val.op==='delete') {
    var index = acc.findIndex((elt) => elt.id === val.id);
    acc.splice(index, 1);
    return acc;
  } else {
    return acc.concat(val);
  }
});

To trigger an element deletion, simply send an event on the subject:

this.deleteSubject.next({op:'delete', id: '1'});

See this plunkr: https://plnkr.co/edit/8bYoyDiwM8pM74BYe8SI?p=preview .

Answer 2

You can take advantage of filter operator:

this.places$
        .pipe(
            map(places => {
                // Here goes some condition, apply it to your use case, the condition only will return when condition matches
                return places.filter(place => place.placeId !== 0);
            }),
            map(response => (this.users$ = of(response)))
        )
        .subscribe(result => console.warn('Result: ', result));

Answer 3

The filter function is immutable and won't change the original array.

I would change the deletePlace function to something like this:-

deletePlace(placeId: number):  void {
  this.apiService.deletePlace(placeId)
  .subscribe(
    (res: any) => {
      this.places = this.places.filter((place) => place.id != placeId);
    }, 
    (err: any) => console.log(err)
  );    
}  

Answer 4

RxJS version 6

Using the accepted answer with RxJS 6 and typescript will throw an error because the observables hold different types. you better use combineLatest , you could also use zip but it will not work! did you just ask why? the answer is here :)

combineLatest([
  this.items$,
  this.deleteItem$
]).pipe(
  takeUntil(this.onDestroy),
  tap(([items, deleteItem]) => {
    if (deleteItem && deleteItem.op === 'deleteItem') {
      var index = items.findIndex((item) => item.id === deleteItem.id);
      if (index >= 0) {
        items.splice(index, 1);
      }
      return items;
    }
    else {
      return items.concat(deleteItem);
    }
  })
).subscribe();

then you can send the event..

this.deleteItem$.next({ op: 'deleteItem', id: '5e88fce485905976daa27b8b' });

I hope it will help someone..