Reducer errors swallowed silently after epics' outputs

[haf]

const reducer = (state = {}, action) => {
  switch (action.type) {
    case 'THING':
      return { ...state, thing: action.nonexistent.childprop }
    default:
      return state;
  }
};
export function sampleEpic(action$: Observable, store: Store) {
  return action$.ofType(ANYTHING)
    .concatMap(action =>
      fetch('/api/anything', { ... })
    .mergeMap(response => {
      switch (response.status) {
        case 409:
          return Observable.of(pleaseNo(email));
        case 201:
          return Observable.of({ type: 'THING' }, { type: 'NEVER_DISPATCHED' });
        default:
          throw new Error(`Unknown response code ${response.code}`);
      }
    });
}

The THING action causes a crash that is hidden from the user (me for the last 2 hours) so that it's very hard to see where things went wrong.

Adding catch to the end of the epic doesn't make a difference.

All middleware a successfully run.

[BerkeleyTrue]

I can't see how redux-observable would catch errors thrown in your reducer. RO does not contain your reducer or any other redux aspect other than the epics themselves. Also, there aren't cases where RxJS would swallow errors like promises would. The default is always to throw errors if unhandled by the user.

I'm also failing to see how the THING action would crash as there are no syntax errors present

 return { ...state, thing: action.nonexistent }

The result of this line of code is a state with the thing key set to undefined.

[haf]

Perhaps it's a redux bug then?

.childprop in sample now.

[jayphelps]

I'm digging into this. Right now my findings are that RxJS did not swallow this error in 5.0.3 but started to in 5.1.0. https://jsbin.com/gehasuy/edit?html,js,output

It's not yet clear if this is a bug in Rx or if we were relying on buggy behavior that was fixed in 5.1.0. My initial (biased) reaction is that Rx should not be swallowing this error condition but it's possible I'm missing something.

Will continue digging. If anyone else wants to do so as well, you just need to pause your debugging and step through to see what happens.

---

It seems to be somehow related to merging into an observable of multiple items Observable.of({ type: 'SECOND' }, { type: 'THIRD' }). Only a single item and it correctly propagates the errors.... Observable.of({ type: 'SECOND' }). 🤔

[jayphelps]

EDIT: this particular situation no longer gets swallowed, but some other cases still do.

---

Minimum reproduction:

https://jsbin.com/dukixu/edit?html,js,output

const input$ = new Rx.Subject();

input$
  .mergeMap(() => Rx.Observable.of(1, 2))
  .subscribe(d => {
    throw new Error('some error');
  });

input$.next();

Also appears related to Subject usage (but could be a red herring)

[jayphelps]

Similar thing was reported and fixed ReactiveX/rxjs#2565 but it did not fix this issue for us. Discussing here: ReactiveX/rxjs#2618

[jayphelps]

This PR is supposed to fix it: ReactiveX/rxjs#2796

[shakyShane]

This is still a bug I'm afraid even on the latest RxJS (5.4.3)

https://jsbin.com/xevuwecojo/1/edit?html,js,console

[jayphelps]

The fix in that PR has not been released (5.4.3 came out before it).