a.js

// Frontend
const calculateBookingTotals = (reservations: ReservationModel[]): BookingTotals => {
  let grossTotal = 0;
  let taxTotal = 0;
  let cityTaxTotal = 0;
  let touristTaxTotal = 0;
  let salesTaxTotal = 0;

  reservations.forEach((reservation) => {
    grossTotal = grossTotal + (reservation.totalGrossAmount.amount || 0);

    if (reservation.extras) {
      const extrasTotal = calculateExtrasTotals(reservation.extras);
      grossTotal = grossTotal + extrasTotal.grossTotal;
      taxTotal = taxTotal + extrasTotal.taxTotal;
    }

    if (reservation.taxes) {
      reservation.taxes.forEach((tax) => {
        if (tax.type === TaxTypeEnum.VAT) {
          taxTotal = taxTotal + tax.amount;
        }
        if (tax.type === TaxTypeEnum.CityTax) {
          cityTaxTotal = cityTaxTotal + tax.amount;
        }
        if (tax.type === TaxTypeEnum.TouristTax) {
          touristTaxTotal = touristTaxTotal + tax.amount;
        }
        if (tax.type === TaxTypeEnum.SalesTax) {
          salesTaxTotal = salesTaxTotal + tax.amount;
        }
      });
    }
  });

  return {
    netTotal: grossTotal - taxTotal,
    taxTotal,
    grossTotal: grossTotal + cityTaxTotal + touristTaxTotal + salesTaxTotal,
    cityTaxTotal,
    touristTaxTotal,
    salesTaxTotal
  }
}

// Server

calculateBookingTotals(reservations: ReservationModel[]) {
  return reservations.reduce((total, reservation) => {
    if (reservation.prePaymentAmount) {
      total.prePaymentAmount = total.prePaymentAmount + reservation.prePaymentAmount.amount;
    }

    if (reservation.extras) {
      reservation.extras.forEach((extra) => {
        if (extra.totalGrossAmount) {
          total.totalGrossAmount = total.totalGrossAmount + extra.totalGrossAmount.amount;
        }

        if (extra.prePaymentAmount) {
          total.prePaymentAmount = total.prePaymentAmount + extra.prePaymentAmount.amount;
        }
      });
    }
    if (reservation.taxes) {
      const taxAmount = reservation.taxes.reduce((totalAmount, tax) => {
        return totalAmount + tax.amount;
      }, 0)
      total.totalGrossAmount = total.totalGrossAmount + taxAmount;
    }

    if (reservation.totalGrossAmount) {
      total.totalGrossAmount = total.totalGrossAmount + reservation.totalGrossAmount.amount;
    }

    return total;
  }, {
    prePaymentAmount: 0,
    totalGrossAmount: 0
  });
}