Angular - Forkjoin after forkjoin

Hi I just recently discovered the power of forkjoins for APIs and trying to use it speed up load times. As the title says seeking help on best practice to run a forkjoin after a forkjoin.

I have a forkjoin which grabs some data for 2 x APIs. Based on the data is grabbed I need to run a 2nd forkjoin for an additional 2 x APIs.

In the first forkjoin for a given id I grab a form's data and UX style data.

const form = this.apiService.getForm(id);
const style = this.apiService.getFormStructure(id);

In the 2nd forkjoin I want to pass the photoIds found in the form data to 2 x APIS:

this.apiService.getPhotoUrls(photoIds)
this.apiService.getPhotoHistory(photoIds)

Here is what I have so far:

let formId: string = '1';
const form = this.apiService.getForm(id);
const style = this.apiService.getFormStructure(id);


forkJoin([form, style])
.subscribe(
 (data:Object[]) => {
   // handle result
   let formData = data[1];
   let formStyle = data[2];
   let photoIds:number[] = formData.photoIds;
   //***What is the best to pass the photo Ids to run the next 2 APIS??****//
 }
);

Thanks so much for the help in advance!

You're almost there. Use a higher order mapping operator like switchMap to switch from one observable to another. You could also skip the explicit formData and formStyle initializations using de-structuring assignment

forkJoin([form, style]).pipe(
  switchMap(([formData, formStyle]) => {
      let photoIds: number[] = formData['photoIds'];
      return forkJoin([
        this.apiService.getPhotoUrls(photoIds)
        this.apiService.getPhotoHistory(photoIds)
      ]);
  })
).subscribe({
  next: ([urls, history]) => {
    // `urls` - response from `apiService.getPhotoUrls(photoIds)`
    // `history` - response from `apiService.getPhotoHistory(photoIds)`
  },
  error: (error: any) => {
    // handle error
  }
});

Here is a brief comparison b/n forkJoin operator and switchMap function.