Window.parent

The Window.parent property is a reference to the parent of the current window or subframe.

If a window does not have a parent, its parent property is a reference to itself.

When a window is loaded in an <iframe>, <object>, or <frame>, its parent is the window with the element embedding the window.

var parentWindow = window.parent;

if (window.parent != window.top) {
  // We're deeper than one down
}

• window.frameElement returns the specific element (such as <iframe>) the window is embedded into.
• window.top returns a reference to the top-level window.