總計NaN的項目

Question

我正在開發一個codecademy.com教程，我們正在構建一個收銀機。我應該制定必要的工具來取消收銀機的最後一筆交易。因此，我加入

1. 一個屬性稱爲lastTransactionAmount
2. 的代碼進一步線到功能「添加」來設置最後的交易金額
3. 從總
減去最後一個事務的函數voidLastTransaction量
4. 然後我在底部測試的空白最後交易功能（這是我想到爲void四個巧克力
5. 然後我試圖添加三個巧克力

當我運行代碼，我得到

您的賬單是NaN

有人能解釋我做錯了嗎？

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

有一個在cashRegister沒有財產itemCost。它應該是this.lastTransactionAmount = itemCost;