How to make Chain multiple RxJS observables subscribe angular 8?

Question

I work on angular 8 i have multiple subscribe

i need to make Chain multiple RxJS for getactivesession

and after that subscribe PostUpload .

What I have tried:




this._dataService.getActiveSessions(flagButton).subscribe(resp => {
          if (resp) {
           
            this._dataService.PostUpload(this.selectedoptions,this.fileToUpload)
          .subscribe(blob => {
          saveAs(blob.body, this.fileToUpload.name +'.xlsx');
           });
          }
        });