exports.payPalPay = functions.https.onCall((data, context) => {
  return pay(data, context)
});

//---

export function pay(data, context): Observable<string> {

  const payReq = ...

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

}