Prototype inheritance. obj->C->B->A, but obj.constructor is A. Why?

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);

This code gives next output:

C
A
true

Why obj.constructor here is A and not C ?

Answer 1

As seen in this code sample, you have to manually reset the .constructor property when using inheritance, or your constructor is being overridden when you call new A() or new B():

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

Here is a working sample: http://jsfiddle.net/93Msp/.

Hope this helps!

Answer 2

To make a clear picture:

In the chain

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

only "new A()" object has property constructor. All the other objects get constructor property from the prototype chain.

In the code:

 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);

So as wrote mellamokb we should overwrite(create, if more precise) constructor property.