Angular : Receive responses in order with the calls

forkJoin gives you a little less code,

const arrayOfFetches = this.contactsAttendance
  .map(attendee => this.practiceService.GetPracticebyDBID(attendee.practiceId) );
Observable.forkJoin(...arrayOfFetches)
  .subscribe((practices: Practice[]) => {
      this.practicesAttendance = practices;
      this.ExportExcelAttendance2();
  });

Edit Snap! @Anas beat me to it. Although, I don't think you need the concatAll()

you should use concatAll operator to ensure calling your observables in sequence.

also, you can use completed callback to call ExportExcelAttendance2 instead of checking practicesAttendance length on every response callback.

check the below example:

let contactsAttendanceObservables = this.contactsAttendance
  .map((item) => {
    return this.practiceService.GetPracticebyDBID(item.practiceId);
  });
Observable.of(...contactsAttendanceObservables)
  .concatAll()
  .subscribe(
    (practice: Practice) => {
      this.practicesAttendance.push(practice);
    },
    (err) => {
      // handle any errors.
    },
    () => {
      // completed
      this.ExportExcelAttendance2();
    }
  );

if you still want your observables to run in parallel, you can use forkJoin Operator, which will emit the last value of all the passed observables to a one subscriber when all observables are completed.

check the below example:

let contactsAttendanceObservables = this.contactsAttendance
  .map((item) => {
    return this.practiceService.GetPracticebyDBID(item.practiceId);
  });
Observable.forkJoin(...contactsAttendanceObservables)
  .subscribe(
    (practices: Practice[]) => {
      this.practicesAttendance = practices;
      this.ExportExcelAttendance2();
    }
  );

The forkJoin operator is simple to use. It waits until all observables complete, then emit an array with all the items emitted.

ExportExcelAttendance() { 
  const all = this.contactsAttendance.map(it => this.practiceService.GetPracticebyDBID(it.practiceId)); 
  Rx.Observable.forkJoin(all) 
    .subscribe( 
      practicesAttendance => this.ExportExcelAttendance2(practicesAttendance), 
      error => this.errorMessage = < any > error); 
}