Get a list, perform a fetch on each item and return one array of objects as result

call-observables-after-observable.js

/**
Returns a list of objects
1. get the list of files to fetch
2. fetch each file to get the object
3. return an array of objects 
*/
getAll(): Observable<Tutorial[]> {
  const list = this.dataSrv.getDataList()
    .pipe(
      // get list from dataList
      map(dataList => dataList.list),
      // merge all observables in one
      mergeMap(list => { 
        // call all observables in parallel
        return forkJoin(list.map(file => { 
          // get object as observable
          return this.dataSrv.getData(file) 
        }))
      })
    )
  return list;
}

/* Simplified */
getAll(): Observable<Tutorial[]> {
  return this.dataSrv.getTutorialsData()
    .pipe(
      map(dataList => dataList.list),
      mergeMap(list => forkJoin(list.map(file => this.dataSrv.getTutorialData(file))))
    )
}