//1.client method to retrieve payment ID
const makePayment = this.cfns.httpsCallable('payPalPay');
const paymentIdResult = makePayment({price: 9});

paymentIdResult.subscribe(val => {
  //val is empty regardless of what I do on the server-side
  return val.paymentId;
});



//2. server endpoint being called
exports.payPalPay = functions.https.onCall((data, context) => {
  return pay(data, context)
});



//3. actual server-side logic
export function pay(data, context): Observable<string> {
    var subject = new Subject();
  const payReq = ...

  paypal.payment.create(payReq, (error, payment) => {
      if (payment.state === 'created') {
        subject.next({paymentId: payment.id});
        subject.complete();
    }
  });
    return subject;

//   tried returning string instead of Observable<string>. No luck
//    subject.subscribe(val =>{
//      return val;
//    });
}