調用字符串后函數返回未定義

Question

我正在嘗試使用JS制作購物車，而我的任務之一就是創建一個placeOrder函數。

• placeOrder（）函數接受一個參數，即信用卡號。

• 如果未收到任何參數，則該函數應打印出“對不起”，我們沒有為您准備的信用卡。

• 如果收到卡號，此功能應打印出Your total cost is $71, which will be charged to the card 83296759 。 然后，它應該清空購物車數組。

但是，當我在total函數中調用字符串時，返回的值始終未定義。

 var cart = []; function getCart() { return cart; } function setCart(c) { cart = c; return cart; } function addToCart(itemName) { var object = { [itemName]: Math.floor(Math.random(1, 100) * 100) }; cart.push(object); console.log(`${itemName} has been added to your cart`); return cart; } function total() { if (cart.length !== 0) { var totalValue = []; for (var i = 0; i < cart.length; i++) { for (var item in cart[i]) { totalValue.push(cart[i][item]); var sum = totalValue.reduce(function(a, b) { return a + b; }, 0); console.log(`The total value is ${sum}`); } } } else { return ("Your shopping cart is empty.") } } function placeOrder(cardNumber) { if (cardNumber === undefined) { return ("Sorry, we don't have a credit card on file for you."); } else { console.log(`Your total cost is $${total()}, which will be charged to the card ${cardNumber}`); cart = []; return cart; } } addToCart("a"); addToCart("be"); addToCart("cart"); placeOrder(14564); 

輸出：

Your total cost is $undefined, which will be charged to the card 14564

Answer 1

當cart.length！= 0時，函數total不會返回。您必須返回sum。

if (cart.length !== 0) {
    var sum = 0;
    for(var i=0; i< cart.length; i++) {
      for(var item in cart[i]) {
        totalValue.push(cart[i][item]);
        sum = totalValue.reduce(function(a, b) {return a + b;}, 0);
        console.log(`The total value is ${sum}`);    
      }
    }
    return sum;
}

Answer 2

正如@Carcigenicate所說，您應該在if（cart.length！== 0）{}語句的結尾處調用return。 請記住，當調用return時，該函數立即停止執行，而不會評估進一步的指令。

Answer 3

您需要確保total總是返回一個number ，例如：

 var cart = [{ "a": 86 }, { "be": 2 }, { "cart": 24 } ]; function total(cart) { return cart .reduce(function(prices, item) { return prices.concat(Object.keys(item).map(k => item[k])); }, []) .reduce(function(sum, price) { return sum += price; }, 0); } console.log(total([])); console.log(total(cart)); 

然后， placeOrder的用法如下所示：

 function total(cart) { return cart .reduce(function(prices, item) { return prices.concat(Object.keys(item).map(k => item[k])); }, []) .reduce(function(sum, price) { return sum += price; }, 0); } function placeOrder(cardNumber, cart) { if (cardNumber === undefined) { return "Sorry, we don't have a credit card on file for you."; } if (cart.length === 0) { return "You cart is empty"; } return `Your total cost is $${total(cart)}, which will be charged to the card ${cardNumber}`; return []; } console.log(placeOrder(1234, [])); console.log(placeOrder(1234, [{ a: 1 }, { b: 2 }])); 

Answer 4

• 我不知道該數組totalValue的目的是什么，但是您不需要該函數reduce ，只需遍歷項目並求和。
• 您正在遍歷每個項目的值，直接使用Object.values(cart[i]).pop();獲得項目的數量Object.values(cart[i]).pop(); 。

---

var sum = 0;
for (var i = 0; i < cart.length; i++) {
  var amount = Object.values(cart[i]).pop();
  sum += amount;        
}

最后，返回該sum 。

 var cart = []; function getCart() { return cart; } function setCart(c) { cart = c; return cart; } function addToCart(itemName) { var object = { [itemName]: Math.floor(Math.random(1, 100) * 100) }; cart.push(object); console.log(`${itemName} has been added to your cart`); return cart; } function total() { if (cart.length !== 0) { var sum = 0; for (var i = 0; i < cart.length; i++) { for (var item in cart[i]) { sum += cart[i][item]; } } console.log(`The total value is ${sum}`); return sum; } return -1; // Could be 0, this is up to you. } function placeOrder(cardNumber) { if (cardNumber === undefined) { return ("Sorry, we don't have a credit card on file for you."); } else { var sum = total(); if (sum) { console.log(`Your total cost is $${sum}, which will be charged to the card ${cardNumber}`); } else { console.log("Your shopping cart is empty.") } cart = []; return cart; } } addToCart("a"); addToCart("be"); addToCart("cart"); placeOrder(14564);