What operators *require* a synchronous error or complete?

[dead-claudia]

Besides onErrorResumeNext, what operators require a synchronous error or complete?

(Stemmed from @zenparsing's response to a quick strawman of mine.)

/cc @Blesh @jhusain

[dead-claudia]

I'm aware that onErrorResumeNext likely requires internal help, anyways.

[acutmore]

'Require' is a tough criteria. From experience using RxJs's implementation of Observables in user interfaces which run on devices with limited resources, maximising synchronous calls is essential to achieving performance. An asynchronous call can't be inlined.

One example where an asynchronous complete would delay getting a value to an observer:

const lastValue = observable =>
  new Observable(observer => {
    const cache = [];
    return observable.subscribe(
      value => cache[0] = value,
      err => observer.error(err),
      () => {
          if (cache.length === 1) {
              observer.next(cache[0]);     
          }
          observer.complete();
      }
    );
 });

[jhusain]

Observables must be able to synchronously dispatch complete and error notifications. See here for a comprehensive explanation. https://docs.google.com/document/d/1Zo-ls61hNviKl-hfLSoR3NeGoIA-Z0cBvQRof5YEzVw

The rationale document above includes several operators which need the ability to dispatch synchronously.

[dead-claudia]

@jhusain Thanks!