对JavaScript原型继承感到困惑 [英] Confused about JavaScript prototypal inheritance

问题描述

在 JavaScript权威指南5版一书中，9.2节原型和继承，我找到以下字样：

在上一节中，我展示了
new运算符创建一个新的空
对象，然后调用构造函数
函数作为该对象的方法。
这不是完整的故事，但是b $ b。创建空的
对象后，new设置该
对象的原型。对象的原型是
其构造函数
的prototype属性的值。所有
函数都有一个自动创建的原型属性
，并且在函数定义为
时初始化
。
prototype属性的初始值是一个具有
单个属性的对象。这个属性是
命名的构造函数，并且返回
构造函数，原型与
相关联。 （你
可能会回想起构造函数属性来自第7章的
;这就是为什么每个
对象都有一个构造函数属性。）
你添加到这个
原型对象的任何属性看起来都是
对象的属性由
初始化构造函数。

现在，如果确实如此，原型继承如何存在？我的意思是，假设构造函数的原型对象最初具有构造函数属性。因为原型对象本身是一个对象，为了确定它的构造函数，我们经常使用 prototype_object.constructor 。但是现在 prototype_object 已经有一个构造函数属性本身，它指向 构造函数与原型相关联的 。在这种情况下，如何存在继承？

解决方案

让我们说，狗是一个哺乳动物。

  function Mammal（）{
 this.milk = true; 
}; 
 
 
 
 function Dog（）{
 this.bark = true; 
} 
 Dog.prototype =新的哺乳动物; 

所以Dog points的原型到哺乳动物的对象。这个Mammal对象有一个对它的构造函数的引用，所以当Dog是新的时，JavaScript看到Dog原型是一个哺乳动物，所以调用Mammal的构造函数来生成一个有效的Mammal对象（另一个）然后使用Dog构造函数使它成为Dog对象。 p>

从这里， Dog.prototype 的构造函数是 Mammal （添加了额外字段和函数的哺乳动物对象） Dog 的构造函数是 Dog 。遗传的存在是因为Dog的一个实例以哺乳动物为原型;因此，狗是一个哺乳动物。当一个方法被调用并且JS无法从 Dog.prototype 中找到它时，JS会查看 Mammal.prototype （这是添加了额外字段和函数的Object。）

希望这会有所帮助。

In the book "JavaScript the definitive guide 5 edition", section 9.2 Prototypes and Inheritance, I find the following words:

In the previous section, I showed that the new operator creates a new, empty object and then invokes a constructor function as a method of that object. This is not the complete story, however. After creating the empty object, new sets the prototype of that object. The prototype of an object is the value of the prototype property of its constructor function. All functions have a prototype property that is automatically created and initialized when the function is defined. The initial value of the prototype property is an object with a single property. This property is named constructor and refers back to the constructor function with which the prototype is associated. (You may recall the constructor property from Chapter 7 ; this is why every object has a constructor property.) Any properties you add to this prototype object will appear to be properties of objects initialized by the constructor.

Now, if that is true, how could prototypal inheritance exists? I mean, let's say the prototype object of a constructor function has a constructor property initially. Because the prototype object itself is an object, to determine its constructor we often use prototype_object.constructor. But now the prototype_object already has a constructor property itself, and it points to the constructor function with which the prototype is associated. In this situation how can inheritance exists?

解决方案

Let say, Dog is a Mammal.

function Mammal() {
  this.milk = true;
};


function Dog() {
  this.bark = true;
}
Dog.prototype = new Mammal;

So prototype of Dog points to an object of Mammal. This Mammal object has a reference to its constructor so when Dog is new, JavaScript see that Dog prototype is a Mammal so Mammal's constructor is called to produce a valid Mammal object (another one) then make it a Dog object using Dog constructor.

From this, the constructor of Dog.prototype is a Mammal (a Mammal Object that has extra fields and functions added) BUT constructor of Dog is Dog. The inheritance exist because the an instance of Dog has a Mammal as a prototype; hence, Dog is a Mammal. When a method is called and JS cannot find it from Dog.prototype, JS look in Mammal.prototype (which is an Object that has extra fields and functions added).

Hope this helps.

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