Angular 9 rxjs - forkJoin returns array of Observables instead of array of values

Question

Here is a simplified version of my problem:

this.store.pipe(
  select(arr),
  switchMap(arr => {
    const data = arr.map(arrItem => this.dataService.getData(arrItem.id));
    return forkJoin(...data);
  }
  map(data => {
    console.log(data);
    return data;
  )
);

Somehow, in console.log I'm getting array of values only if arr.length = 1. If arr.length > 1, I'm getting array of Observables instead.

How is it possible? Am I misunderstanding forkJoin? How can I fix it?

What is the response type of this.dataService.getData method ? An Observable from an HttpClient.get request ? — Thierry Falvo
– Thierry Falvo, Commented Jul 7, 2020 at 14:38

Thierry Falvo · Accepted Answer · 2020-07-07 15:10:54Z

1

If your service response is an observable and you want to retrieve an array of response at the end, you can do this :

this.store.pipe(
  select(arr),
  switchMap(arr => from(arr)),
  mergeMap(arrItem => this.dataService.getData(arrItem.id), 2),  // max 2 concurrents requests here
  toArray()
).subscribe(datas => {
  // datas is an array of returned data from getData service
});

Note here second argument of mergeMap (= 2), to specify maximum concurrent request.

switchMap(arr => from(arr)) : from an array value, to a new observable with emitted values = each value of the array

toArray() : take all values emitted, and transform to one array

edited Jul 7, 2020 at 15:10

answered Jul 7, 2020 at 14:56

Thierry Falvo

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1 Comment

Jaruzel Over a year ago

Thanks for the response, I'm now trying to implement it to my code. Will update you when I'm done.

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