原型继承。 obj->C->B->A，但是obj.constructor是A。为什么？

Question

var print = function(text){
  document.write(text);
  document.write("</br>");
}

var A = function(){
}
A.prototype.name="A";

var B = function(){
}
B.prototype = new A();
B.prototype.name="B";

var C = function(){
}
C.prototype = new B();
C.prototype.name="C";

obj = new C();
print(obj.name);
print(obj.constructor.prototype.name);
print(obj.constructor == A);

这段代码给出了下一个输出：

C
A
true

为什么这里的 obj.constructor 是 A 而不是 C？

Answer 1

如this code sample 中所见，使用继承时您必须手动重置.constructor 属性，否则当您调用new A() 或new B() 时您的构造函数被覆盖：

B.prototype = new A();
B.prototype.constructor = B; // need this line to fix constructor with inheritance

这是一个工作示例：http://jsfiddle.net/93Msp/。

希望这会有所帮助！

Answer 2

为了清晰的画面：

在链中

 obj->"new B()"->"new A()"   // where obj is the same as "new C()"

只有"new A()" 对象具有constructor 属性。所有其他对象都从原型链中获得constructor 属性。

在代码中：

 var A = function(){
 }
 A.prototype.name="A";
 // A had not create "constructor" property for "new A()" 
 // so, looking at the prototype
 // According to ECMAScript spec 13.2.10
 // A.prototype.constructor == A
 // thus
 // "new A()".constructor == A

 var B = function(){
 }
 B.prototype = new A();
 B.prototype.name="B";
 // B had not create "constructor" property for "new B()" 
 // looking at the prototype
 // B.prototype is "new A()" 
 // as shown above 
 // "new A()".constructor == A
 // thus
 // B.prototype.constructor == A
 // and at the end
 // "new B()".constructor == A

 var C = function(){
 }
 C.prototype = new B();
 C.prototype.name="C";
 // C had not create "constructor" property for "new C()"/"obj" 
 // looking at the prototype
 // C.prototype is "new B()" 
 // looking up to "new B()".prototype
 // "new B()".prototype is "new A()" as shown above
 // "new A()".constructor == A
 // and finally
 // C.prototype.constructor == A

 obj = new C();
 print(obj.name);
 print(obj.constructor.prototype.name);
 print(obj.constructor == A);

所以正如 mellamokb 所写，我们应该覆盖（创建，如果更准确的话）constructor 属性。

Answer 3

我认为这篇文章很好地解释了原型继承中的构造函数

http://phrogz.net/js/classes/OOPinJS2.html