Angular Observable destroy with takeUntil: What happens when .next() is missing in ngOnDestroy

Question

In an Angular 7 Component, I use the RxJS takeUntil() to properly unsubscribe on observable subscriptions.

• What happens when the this.destroy$.next() is missing in the method ngOnDestroy (see sample below)? Will it still unsubscribe properly?
• What happens when the this.destroy$.complete() is missing in the method ngOnDestroy (see sample below)? Will it still unsubscribe properly?
• Is there any way to enforce that pattern with takeUntil() to unsubscribe is properly used (eg tslint rule, npm package)?

@Component({
    selector: 'app-flights',
    templateUrl: './flights.component.html'
})
export class FlightsComponent implements OnDestroy, OnInit {
    private readonly destroy$ = new Subject();

    public flights: FlightModel[];

    constructor(private readonly flightService: FlightService) { }

    ngOnInit() {
        this.flightService.getAll()
            .pipe(takeUntil(this.destroy$))
            .subscribe(flights => this.flights = flights);
    }

    ngOnDestroy() {
        this.destroy$.next();
        this.destroy$.complete();
    }
}

Answer 1

1. takeUntil takes next as emission. If only complete() called it won't unsubscribe

try this out:

const a=new Subject();
interval(1000).pipe(takeUntil(a)).subscribe(console.log);
timer(3000).subscribe(_=>a.complete())

2. this.destroy$ is still in the memory and won't get garbage collected
3. Not that i am aware of

Please also take a look here to avoid memory leak when using takeUntil .

https://medium.com/angular-in-depth/rxjs-avoiding-takeuntil-leaks-fb5182d047ef

Personally I prefer explicit unsubscribe upon destroy.

Answer 2

this.destroy$.next() triggers the Subject which triggers the takeUntil operator which completes the flightService.getAll() subscription.

this.destroy$.complete() completes the destroy$ Subject when component is destroyed.

Answer 3

Hi use takeWhile instead of takeUntil .

@Component({
    selector: 'app-flights',
    templateUrl: './flights.component.html'
})
export class FlightsComponent implements OnDestroy, OnInit {
    private readonly destroy$ = new Subject();
    alive = true;
    public flights: FlightModel[];

    constructor(private readonly flightService: FlightService) { }

    ngOnInit() {
        this.flightService.getAll()
            .pipe(takeWhile(() => this.alive))
            .subscribe(flights => this.flights = flights);
    }

    ngOnDestroy() {
        this.alive = false;
        this.destroy$.complete();
    }
}

Answer 4

Here's a simpler approach:

private readonly destroy$ = new Subject<boolean>();

...

ngOnDestroy() {
    this.destroy$.next(true);
    this.destroy$.unsubscribe();
}