Question

I am having a little trouble understanding why Result is false in the following code.

Other objects in the library that are required to have a size will check if the size parameter is an instance of InterfaceKit.Core.Size. Currently, instanceof is returning false.

var InterfaceKit = {
    Core : {
        Size: function( i_Width, i_Height ){
            Object.defineProperties(this, {
                m_Width : {
                    value: Number( i_Width ) ? Number( i_Width ) : 0
                    , writable: true
                }
                , m_Height : {
                    value: Number( i_Height ) ? Number( i_Height ) : 0
                    , writable: true
                }

            }); 

            this.__proto__ = {
                SetWidth: function( i_Width ){
                    if( Number( i_Width ) )
                        this.m_Width = Number( i_Width ); 

                }
                , GetWidth: function(){
                    return this.m_Width; 
                }
                , SetHeight: function( i_Height ){
                    if( Number( i_Height ) )
                        this.m_Height = Number( i_Height ); 

                }
                , GetHeight: function(){
                    return this.m_Height; 
                }

            };

            this.__proto__.constructor = InterfaceKit.Core.Size; 

        }

    }

}; 

var Result = (new InterfaceKit.Core.Size( 10, 10 ) instanceof InterfaceKit.Core.Size); //false

Answer 1

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/instanceof

The instanceof operator tests whether an object has in its prototype chain the prototype property of a constructor.

In other words…

// an objects prototype chains has this as the root
object.__proto__ // set to: {}

// the prototype property of the constructor
A.B.prototype // unmodified prototype, some object set by JS engine. Probably: {}

// Equivalent objects are not equal, because they are different objects
{} === {} // false

Therefore the constructor’s prototype is not in the prototype chain of the object. Which means it is not an instanceof the constructor, even if that constructor did “construct” that object.

---

But really, just don’t use __proto__.

var A = {
    B: function() {}
};
A.B.prototype = {};

var Result = (new A.B() instanceof A.B); // true