在字符串中调用后返回未定义的函数

Question

我正在尝试使用 JS 制作购物车，我的任务之一是创建 placeOrder 函数。

• placeOrder() 函数接受一个参数，即信用卡号。

• 如果没有收到参数，该函数应打印出对不起，我们没有为您存档的信用卡。

• 如果收到卡号，该函数应打印出Your total cost is $71, which will be charged to the card 83296759。然后，它应该清空购物车数组。

但是，当我将总函数调用到字符串中时，会一直返回 undefined。

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);

输出：

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

Answer 1

当 cart.length != 0 时，函数 total 不返回。 你必须返回总和。

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 所说，您应该在 if(cart.length !== 0) {} 语句的末尾调用 return 一些东西。请记住，当调用 return 时，该函数会立即停止执行，而不会对进一步的指令进行评估。

Answer 3

• 我不知道该数组 totalValue 的用途是什么，但您不需要该函数 reduce，只需遍历项目并求和即可。
• 您正在遍历每个项目的值，直接使用Object.values(cart[i]).pop(); 获取您的项目数量。

---

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

最后，返回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);

Answer 4

您需要确保 total 始终返回 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));

placeOrder 中的用法可能如下所示：

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
}]));