Observables & HTTP — Series 3

Preview — 3 of 10 questions

Does anything related to numbers$s values get logged or computed before its subscribed to?

javascript

const numbers$ = of(1, 2, 3);
console.log('created');
// no .subscribe() call yet

ANo — Observables are lazy by default. Nothing inside an Observable's definition actually runs until something calls .subscribe() on it; numbers$ here is just a description of a future stream of values, not yet an active one

BYes — of(1, 2, 3) evaluates immediately when called, the values just aren't delivered anywhere yet

CIt depends on whether the Observable is "hot" or "cold" — of() specifically is always hot

DOnly the first value (1) is computed eagerly; the rest wait for subscription

For a source emitting 5, what gets logged, and in what order?

javascript

source$.pipe(
  tap(value => console.log('saw', value)),
  map(value => value * 2),
).subscribe(result => console.log('got', result));

Asaw 5, then got 10 — tap runs a side effect (here, logging) with the value passing through completely unchanged, before the rest of the pipeline (map) transforms it further

Bgot 10 only — tap is a no-op unless its result is used

Csaw 10, then got 10 — tap always sees the final, fully transformed value regardless of where it sits in the pipe

Dgot 10, then saw 5 — tap runs after the subscriber's callback

What's the practical difference between these two?

javascript

interval(1000).subscribe(n => console.log('interval', n));
timer(3000, 1000).subscribe(n => console.log('timer', n));

AThey're identical; timer is just an older, deprecated name for interval

Binterval(1000) emits 0, 1, 2, ... every second, starting immediately (the first emission is after one second). timer(3000, 1000) waits 3000ms before its first emission, then also emits every 1000ms after that — timer lets you control the initial delay separately from the repeat period, which interval doesn't

Cinterval only emits once; timer repeats indefinitely

Dtimer requires an explicit .subscribe() argument specifying how many emissions to allow

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