Window: top property
Returns a reference to the topmost window in the window hierarchy.
Value
The reference to the topmost window.
Notes
Where the window.parent property returns the immediate parent of the
current window, window.top returns the topmost window in the hierarchy of
window objects.
This property is especially useful when you are dealing with a window that is in a subframe of a parent or parents, and you want to get to the top-level frameset.