How to return value inside subscribe Angular 4

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))

Observable runs when you subscribe to it and what return from subscription is always a subscription object like setInterval. first sample: because this is a async call, second console.log won't wait for subscribe to finish before it executes.

getTotalQuestions(idForm: string) {
  let totalQuestions: number;
  return this.getFirebaseData(idForm + "/Metadatos").pipe(
     map(items =>
      items.map(item => {
        totalQuestions = item.Total;
        console.log(totalQuestions);
      });
    ));
}

getTotalQuestions('231').subscribe(console.log)