How to return value inside subscribe Angular 4

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You can’t directly return totalQuestions like that, you need to use a subject to achieve that.

getTotalQuestions(idForm:string): Observable<string> {
let totalQuestions:number;
var subject = new Subject<string>();
this.getFirebaseData(idForm+"/Metadatos")
.subscribe(items => {
    items.map(item => {

      totalQuestions=item.Total;
      console.log(totalQuestions);
      subject.next(totalQuestions);
    });
  }
);
  return subject.asObservable();
}

Usage: getTotalQuestion(idForm).subscribe((r)=>console.log(r))

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