Any need to call unsubscribe for RxJS first()

Question

In the following code:-

RxJS.Observable.of(1,2).first().subscribe((x) => console.log(x););

is it necessary to unsubscribe given the operator first()?

Harwilll

Answer 1

No. It unsubscribes automatically after calling first(). The current syntax is observable.pipe(first()).subscribe(func); for RxJS 6.

The documentation states:

If called with no arguments, first emits the first value of the source Observable, then completes.

Enterprising

Think you are missing a ) :) – Nedrud

what happens when the source observable never emits a value, does that mean that the Subscription stays there and possibly create a memory leak ? – Cripps

@Cripps Yes, it can create a memory leak. – Hemp

So how to add unsubscribing in this case? – Monseigneur

takeUntil(this.destroyed$) Where destroyed$ gets set on ngOnDestroy() – Gratulant

Answer 2

For the example provided, you dont need to unsubscribe, and neither you need to call first, as Observable.of(1) actually completes after emitting its first (and last) value.

Pachalic

Changed to 1,2 but your answer still is correct since you mentioned it Completes. – Harwilll

Answer 3

first() will complete after the first item is emitted from the observable.

Also subscribe() takes three arguments, the last one being the complete callback. Running the following code will output 1 followed by 'done'.

Rx.Observable.of(1)
  .subscribe(
  (x) => console.log(x),    // next
  (x) => console.error(x),  // error
  () => console.log('done') // done
)

Kroo