Getting a reference to the object that holds a property that is a constructor

A function used as a constructor has only a reference to the new instance, inheriting from its prototype.

To make it maintain a reference to the original A instance, you will need to put the B constructor in a closure:

function A() {
    var that = this;
    this.pa = { x: 1 };

    this.B = function() {
        this.pb = that.pa;
    };
};

var a = new A ();
var b = new a.B ();
console.log (b.pb.x); // does print 1
a.pa.x = 2;
console.log (b.pb.x); // does print 2

However, this has the disadvantage of creating a new B constructor (with its own prototype object) for every single A instance. Better would be something like

function A() {
    this.pa = { x: 1 };
}
A.B = function() {
    this.pb = null;
};
A.prototype.makeB = function() {
    var b = new A.B();
    b.pb = this.pa;
    return b;
};
// you can modify the common A.B.prototype as well

var a = new A ();
var b = a.makeB();
console.log (b.pb.x); // does print 1
a.pa.x = 2;
console.log (b.pb.x); // does print 2

Yet, we could mix the two approaches so that you have only one prototype but different constructors:

function A() {
    var that = this;
    this.pa = { x: 1 };

    this.B = function() {
        this.pb = that.pa;
    };
    this.B.prototype = A.Bproto;
}
A.Bproto = {
    …
};

var A = function (){
    this.pa = { x: 1 };
};

A.prototype.B = function (a){

     this.pb = a.pa;
};
var a = new A ();

var b = new  a.B(a);
console.log(b.pb.x); //should print 1
a.pa.x = 2;
console.log(b.pb.x);

Well, it's not exactly what I want but it's very close:

var A = function (pa){
    this.pa = pa;
};

A.prototype.B = function (a){
    if (this instanceof A.prototype.B){
        if (!a) throw "error";
        this.pb = a.pa;
        return;
    }
    return new A.prototype.B (this);
};

var a = new A ({ x: 1 });
var b = a.B ();
console.log (b.pb.x); //1
a.pa.x = 2;
console.log (b.pb.x); //2

new a.B () //throws "error"