Observables & HTTP — Series 2

Preview — 3 of 10 questions

The network tab shows nothing. Why?

javascript

ngOnInit() {
  this.http.get<Hero[]>('/api/heroes');
}

AThe URL needs a leading http://

BHttpClient observables are cold: the request is only made when something subscribes, and nothing here does

Cget() returns a promise that must be awaited

DThe call needs to run inside NgZone.run

What is the third callback for?

javascript

source$.subscribe({
  next: (v) => console.log(v),
  error: (e) => console.error(e),
  complete: () => console.log('done'),
});

Acomplete runs after every next

Bcomplete runs whether the stream ended successfully or with an error

Ccomplete runs once when the stream ends normally — and never if it errored, since error and complete are mutually exclusive terminal events

Dcomplete is called when the subscription is unsubscribed

The source completes without emitting anything. What happens with each?

javascript

source$.pipe(take(1))            // 0 or 1 values, always completes
source$.pipe(first())            // errors on an empty source
source$.pipe(first(null, 'n/a')) // …unless a default is supplied

Atake(1) completes silently; first() errors with EmptyError — unless given a default value

BBoth error

CBoth complete silently

Dtake(1) errors; first() completes silently

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