JavaScript 函數返回未定義 + 控制台日志

Question

有人可以幫我修復我的千樹（）函數，它顯示我想要的控制台日志，但它也在控制台上返回 undefined 。

我如何使該功能只執行控制台日志？

這是我正在學習的課程練習的一部分。

該功能就在代碼的底部

謝謝你

// park constructor
class Park {
    constructor (name, buildYear, trees, size) {
        this.name = name;
        this.buildYear = buildYear;
        this.trees = trees;
        this.size = size;
    }

    // number of tree / park area
    treeDensity() {
        return this.trees / this.size;
    }

    // return the park age
    parkAge() {
        return new Date().getFullYear() - this.buildYear;
    }
}

// street constructor
class Street {
    constructor (name, buildYear, size) {
        this.name = name;
        this.buildYear = buildYear;
        this.size = size;
    }
};


// all parks

let allParks = [
    park1 = new Park("Brighton Park", 1900, 1200, 3),
    park2 = new Park("Worthing Park", 1800, 650, 2),
    park3 = new Park("Shoreham Park", 1850, 350, 2)
]
// all streets

let allStreets = [
    street1 = new Street("Brighton Street", 1858, 2000),
    street2 = new Street("Worthing Street", 1950, 1200),
    street3 = new Street("Shoreham Street", 1850, 800),
    street4 = new Street("Lancing Street", 1980, 760)
];

// calculate the average age of all parks
let parkAgeAvg = function() {
    return (park1.parkAge() + park2.parkAge() + park3.parkAge()) / 3
};
// Display the name of the park with more than 1000 trees
let thousandTrees = function() {
    for (let cur of allParks) {
        if (cur.trees >= 1000) {
          console.log(`${cur.name} has more than 1000 trees, we have verified there is actually ${cur.trees} trees in total.`);
        }
    }
};

// total length of streets and avg length

//size classification of all streets

Answer 1

在 chrome 中，我在 console.log 中沒有看到任何錯誤

 // park constructor class Park { constructor (name, buildYear, trees, size) { this.name = name; this.buildYear = buildYear; this.trees = trees; this.size = size; } // number of tree / park area treeDensity() { return this.trees / this.size; } // return the park age parkAge() { return new Date().getFullYear() - this.buildYear; } } // street constructor class Street { constructor (name, buildYear, size) { this.name = name; this.buildYear = buildYear; this.size = size; } }; // all parks let allParks = [ park1 = new Park("Brighton Park", 1900, 1200, 3), park2 = new Park("Worthing Park", 1800, 650, 2), park3 = new Park("Shoreham Park", 1850, 350, 2) ] // all streets let allStreets = [ street1 = new Street("Brighton Street", 1858, 2000), street2 = new Street("Worthing Street", 1950, 1200), street3 = new Street("Shoreham Street", 1850, 800), street4 = new Street("Lancing Street", 1980, 760) ]; // calculate the average age of all parks let parkAgeAvg = function() { return (park1.parkAge() + park2.parkAge() + park3.parkAge()) / 3 }; // Display the name of the park with more than 1000 trees let thousandTrees = function() { for (let cur of allParks) { if (cur.trees >= 300) { console.log(`${cur.name} has more than 1000 trees, we have verified there is actually ${cur.trees} trees in total.`); } } }; thousandTrees();

Answer 2

您看到的undefined只是因為您在控制台中調用的函數返回undefined ，並且您在控制台中評估的任何 JS 都會顯示表達式的值。 它與控制台日志代碼無關。

旁注，您如何初始化事物存在問題：

let allParks = [
    park1 = new Park("Brighton Park", 1900, 1200, 3),
    park2 = new Park("Worthing Park", 1800, 650, 2),
    park3 = new Park("Shoreham Park", 1850, 350, 2)
]

在某些模式下，JavaScript 會對此park1窒息，因為park1 、 park2和park3是未定義的。 我相信你真正想要的只是：

let allParks = [
    new Park("Brighton Park", 1900, 1200, 3),
    new Park("Worthing Park", 1800, 650, 2),
    new Park("Shoreham Park", 1850, 350, 2)
]

在前者中，您使用賦值表達式的結果填充數組。 在第二個中，你用構造的對象填充它。

allStreets反饋相同。

Answer 3

定義您在數組中使用的變量：

// all parks
let park1,park2,park3;

let allParks = [
     park1 = new Park("Brighton Park", 1900, 1200, 3),
     park2 = new Park("Worthing Park", 1800, 650, 2),
     park3 = new Park("Shoreham Park", 1850, 350, 2)
]
// all streets
let street1, street2, street3, street4;

let allStreets = [
    street1 = new Street("Brighton Street", 1858, 2000),
    street2 = new Street("Worthing Street", 1950, 1200),
    street3 = new Street("Shoreham Street", 1850, 800),
    street4 = new Street("Lancing Street", 1980, 760)
];