获取实例化构造函数的调用对象？

Question

I'm not sure if the title did this justice, but here is what I have...

function A() {};

A.prototype.B = function(){
   this.backref = //??  --  should be set to "a"
};

var a = new A();  // specialized factory...

var b = new a.B();  // instantiate an "a" specialized version of B
var zztop = b.backref instanceof A; //would be true

I need to assign backref to the instance "a" that called the B constructor. How can I do this? I've looked through all the properties in Chrome debugger, and am not even sure it is possible. Anyone know how to do this? I need to do this because I need access to variables stored in "a". Or is there a better way to do something similar?

Answer 1

I'm not sure if there's a way to do this just using the properties given to you, but you can always make the backref one of the arguments to the B constructor:

A.prototype.B = function(backref) {
    this.backref = backref;
}

var a = new A();
var b = new a.B(a);

Ugly, but it works.

Answer 2

What you are describing is called inheritance. You want object B to inherit the properties of object A.

function A() {
  this.length = 3;
}

function B() {
  this.height = 5;
}

B.prototype = new A();
B.prototype.constructor = B;

var obj = new B();

obj.height;  // returns 5
obj.length;  // returns 3

Here's a jsFiddle: http://jsfiddle.net/d6QeT/