Operators & Subjects — Series 2

Preview — 3 of 10 questions

After one failed request, the search box stops working entirely. Why?

javascript

results$ = this.query$.pipe(
  switchMap((q) => this.http.get<Hero[]>(`/api/search?q=${q}`)),
  catchError(() => of([])),
);

AcatchError must be the first operator in the chain

Bof([]) completes, and a completed inner stream completes the outer one

CThe error propagates up to the outer stream, and catchError there replaces the whole chain — including query$ — with of([]), which emits once and completes. Catching inside the switchMap keeps the outer stream alive

DswitchMap cannot be combined with catchError

A save button emits clicks; a form emits values. Which operator gives on click, take the current form value?

javascript

this.clicks$.pipe(
  withLatestFrom(this.form.valueChanges),
  map(([, value]) => value),
  switchMap((value) => this.api.save(value)),
);

AcombineLatest([clicks$, form$])

Bclicks$.pipe(withLatestFrom(form$)) — the click drives emission, the form only contributes its latest value

Cmerge(clicks$, form$)

Dzip(clicks$, form$)

What does each emit for a stream of 1, 2, 3 that then completes?

javascript

const state$ = actions$.pipe(scan((state, action) => reducer(state, action), initial));

Ascan emits 6; reduce emits 1, 3, 6

BBoth emit 1, 3, 6

CBoth emit 6

Dscan emits the running total after each value — 1, 3, 6 — while reduce emits only the final 6, on completion

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