.isPrototypeOf（）和.hasOwnProperty（）方法混淆

Question

假设我有这个代码：

 // Male will inherit ALL of the Human properties function Human(x, y) { // Following properties will be inherited this.name = x; this.age = y; this.test = "Test 1"; } // Following properties will ALSO be inherited Human.prototype.citizen = "USA"; Human.prototype.employer = "Google"; Human.prototype.test = "Test 2"; function Male(x, y) { // Following properties will be the own properties of Male instances this.name = x; this.age = y; this.gender = "Male"; } // Inheritance - Connecting Male object with Human object Male.prototype = new Human(); // no arguments are passed Male.prototype.constructor = Male; // correcting constructor property var albert = new Male("Albert", 25); 

然后，我想对代码进行一些测试

Human.isPrototypeOf(albert); // I expect it to return TRUE

但它返回FALSE，为什么呢？

并且，对于以下测试

Human.hasOwnProperty("age"); // /i expect it to return TRUE

但它返回FALSE，为什么呢？

谢谢，

编辑我的问题与其他问题略有不同，因为它也谈论原型链。

Answer 1

Human功能不在 albert的原型链中。 Human.prototype引用的对象是：

Human.prototype.isPrototypeOf(albert); // true

Human.hasOwnProperty（ “时代”）; //我希望它返回TRUE

Human函数没有age属性。 用new创建的实例：

new Human().hasOwnProperty("age"); // true

---

旁注：您设置继承链的方式很常见，并在很多示例中显示，但在两个方面不正确：

1. 您不想使用new Human来创建Male.prototype 。

2. 你想叫Human从Male ：

所以：

function Male(x, y) {
    // Give Human its chance to initialize the object (#2)
    Human.call(this, x, y);
    // ...
}

// Don't use new Human to create the prototype (#1)
Male.prototype = Object.create(Human.prototype);
Male.prototype.constructor = Male;

你不使用new Human为Male创建原型的原因很简单： Human期望参数，但你没有任何东西可以给它。

这是更新的ES5及该代码的早期版本：

 function Human(name, age) { // Argument names should be meaningful this.name = name; this.age = age; this.test = "Test 1"; } Human.prototype.citizen = "USA"; Human.prototype.employer = "Google"; Human.prototype.test = "Test 2"; function Male(name, age) { Human.call(this, name, age); this.gender = "Male"; } Male.prototype = Object.create(Human.prototype); Male.prototype.constructor = Male; var albert = new Male("Albert", 25); console.log(Human.prototype.isPrototypeOf(albert)); // true console.log(new Human().hasOwnProperty("age")); // true 

或者当然，使用ES2015 +（如果您的目标尚不支持，请进行转换）：

 // THIS SNIPPET REQUIRES A BROWSER WITH ES2015+ SUPPORT class Human { constructor(name, age) { this.name = name; this.age = age; this.test = "Test 1"; } } Human.prototype.citizen = "USA"; Human.prototype.employer = "Google"; Human.prototype.test = "Test 2"; class Male extends Human { constructor(name, age) { super(name, age); this.gender = "Male"; } } let albert = new Male("Albert", 25); console.log(Human.prototype.isPrototypeOf(albert)); // true console.log(new Human().hasOwnProperty("age")); // true 

---

你已经说过你正在试图看看链是如何运作的。 这是创建albert后我们在内存中的图表（为简单起见，删除了一些细节）：

+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+
         |                                                                      |
         \ +−−−−−−−−−−−−−−−−+                                                   |
Human−−−−−>|    function    |                                                   |
           +−−−−−−−−−−−−−−−−+                            +−−−−−−−−−−−−−−−−−−−−+ |
           | prototype      |−−−−−−−−−−−−−−−−−−−−−−−−−−−>|       object       | |    
           | name: "Human"  |                          / +−−−−−−−−−−−−−−−−−−−−+ |    
           +−−−−−−−−−−−−−−−−+                          | | constructor        |−+
                                                       | | citizen: "USA"     |
         +−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ | | employer: "Google" |
         |                                           | | | test: "Test 2"     |
         \ +−−−−−−−−−−−−−−−−+                        | | +−−−−−−−−−−−−−−−−−−−−+
Male−−−−−−>|    function    |                        | |
           +−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−+ | |
           | prototype      |−−−>|     object      | | |
           | name: "Male"   |  / +−−−−−−−−−−−−−−−−−+ | |
           +−−−−−−−−−−−−−−−−+  | | constructor     |−+ |
                               | | [[Prototype]]   |−−−+
           +−−−−−−−−−−−−−−−−+  | +−−−−−−−−−−−−−−−−−+
albert−−−−>|     object     |  |
           +−−−−−−−−−−−−−−−−+  |
           | name: "Albert" |  |
           | age: 25        |  |
           | gender: "Male" |  |
           | [[Prototype]]  |−−+
           +−−−−−−−−−−−−−−−−+

[[Prototype]]是规范用于包含对其原型对象的引用的对象的“内部槽”的名称。 相比之下， prototype ，函数上的属性（例如， Human.prototype ），只是函数的一个普通属性，它指向new将用作它创建的新对象的[[Prototype]]的对象，如果你使用它功能与new 。

该图表上的一些注释：

• 所有函数都有一个[[Prototype]]内部插槽，指向Function.prototype指向的对象（为简单起见，上面省略）。
• Human ，这个函数，有一个name属性： "Human"
• Male ，功能，有一个name属性： "Male"
• 对象albert指的是一个name属性： "Albert"
• albert的[[Prototype]]是Male.prototype ; Male.prototype的[[Prototype]]是Human.prototype （和Human.prototype的[[Prototype]] ，未显示，是`Object.prototype）。

你在评论中说过：

我只是无法理解为什么继承语句后我们可以做Male.prototype.isPrototypeOf(albert)返回true而不是Human.isPrototypeOf(albert) （它返回false ）因为Male.prototype是Human一个实例

因为Human ，这个功能，在albert的原型链中是无处可去的。 让我们看看albert的原型链：

• albert的[[Prototype]]是Male.prototype
• Male.prototype的[[Prototype]]是Human.prototype
• Human.prototype的[[Prototype]]是Object.prototype
• Object.prototype的[[Prototype]]为null

如图：

+−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−−−−−+ 
|    albert     |    | Male.prototype |    | Human.prototype |    |  Object.prototype   |
+−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−−−−−+ 
| [[Prototype]] |−−−>| [[Prototype]]  |−−−>| [[Prototype]]   |−−−>| [[Prototype]]: null |
+−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−+    +−−−−−−−−−−−−−−−−−−−−−+

因此， Human ，这个功能，在这个链条中无处可去。