basket.js

function Basket(customer, items){

    this.add = function(item) {
        items.push(item);
    }

    this.checkout = function(){
        const payments = new PayPalPayments();
        return payments.pay(this.total(), customer.creditCard)
    }

    this.total = function() {
        return items.reduce((sum, item) => sum + (item.price * item.quantity), 0);
    }
}