总计NaN的项目

Question

我正在开发一个codecademy.com教程，我们正在建立一个收银机。我应该制作必要的工具来取消收银机的最后一笔交易。因此，我添加了

1. 一个名为lastTransactionAmount的属性
2. 函数'add'的另一行代码，用于设置最后一笔交易金额
3. 函数voidLastTransaction数量，用于从总计
中减去最后一个事务
4. 然后我测试了底部的void last transaction函数（我希望这四个巧克力无效
5. 然后我尝试添加三个巧克力

当我运行代码时，我得到了

  

你的账单是NaN

有人可以解释我做错了什么吗？

var cashRegister = {
    total:0,
    //Dont forget to add your property
    lastTransactionAmount: 0,         //this is a property I added

    add: function(itemCost) {
        this.total +=  itemCost;
        this.lastTransactionAmount = this.itemCost; //I added this to save 
    },
    scan: function(item,quantity) {
        switch (item) {
        case "eggs": this.add(0.98 * quantity); break;
        case "milk": this.add(1.23 * quantity); break;
        case "magazine": this.add(4.99 * quantity); break;
        case "chocolate": this.add(0.45 * quantity); break;
        }
        return true;
    },
    //Add the voidLastTransaction Method here // I made this function to void l
    voidLastTransaction: function() {
        this.total = this.total - this.lastTransactionAmount; 
    }
};

cashRegister.scan('eggs',1);
cashRegister.scan('milk',1);
cashRegister.scan('magazine',1);
cashRegister.scan('chocolate',4);

//Void the last transaction and then add 3 instead
cashRegister.voidLastTransaction();              //I'm voiding last transaction
cashRegister.scan('chocolate', 3);             //trying to add 3 (instead of 4)chocolate

//Show the total bill
console.log('Your bill is '+cashRegister.total);

Answer 1

itemCost中没有属性cashRegister。它应该是this.lastTransactionAmount = itemCost;