The closed property of the Window object is a read-only Boolean value that specifies whether the window has been closed. When a browser window closes, the Window object that represents it does not simply disappear. The Window object continues to exist, but its closed property is set to true.
Once a window has been closed, you should not attempt to use or manipulate it in any way. If your code needs to use a window that may be closed without your program's knowledge, then you should be sure to always test the closed property before using the Window object.