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window is the global object, so properties on it can be accessed as if they were standalone variables. So referencing top on the top level is the same as referencing window.top . So the question boils down to the difference is between

window.top
window.top.window
window.top gives you a window object (it may be the same as window, or it may be an outer window, if you're in an iframe), and a window's window property is a reference to the same window object (it's a weird self-reference), so those two references are exactly the same as well. You can count on
window.top === window.top.window
to always be true.
                Very informative, thanks! So the second part of my question would be, why? Why can window reference itself? As it is also possible to do window.window and window.window.window.window and so on.
– lukehillonline
                Dec 6, 2019 at 12:28
                MDN says: The point of having the window property refer to the object itself, was likely to make it easy to refer to the global object. Otherwise, you'd have to do a manual var window = this; assignment at the top of your script. Yes, you can chain windows infinitely, just like obj = {}; obj.obj = obj; obj.obj.obj.obj.obj === obj will give true
– CertainPerformance
                Dec 6, 2019 at 12:31
        
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