Why are myarray instanceof Array and myarray.constructor === Array both false when myarray is in a frame?

Question

So the following code alerts false twice:

window.onload = function(){
                    alert(window.myframe.myarray instanceof Array);
                            


        

Answer 1

function isArray(o) {
  return Object.prototype.toString.call(o) === '[object Array]';
}

Long explanation here about why .constructor fails with frames.

The problems arise when it comes to scripting in multi-frame DOM environments. In a nutshell, Array objects created within one iframe do not share [[Prototype]]’s with arrays created within another iframe. Their constructors are different objects and so both instanceof and constructor checks fail:

Answer 2

The two windows each create their own global script environment.

The Array constructor of one is not the same object as the other.

var win2=window.myframe;
alert(win2.myarray instanceof win2.Array); returns true