Is Window Close-able?

I need to know whether `window.close()` is actually going to close the window. This is not a duplicate of Close window - How to determine how window was opened? because that solution relies on checking `window.name`, which is not exactly foolproof; the popup could be opened with a different name but still be close-able via `window.close()`. I can't just check whether `window.opener` is defined or not null because in Firefox and Chrome, if the user closes the opener, `window.opener` gets set to `null`. For example: ``` // In parent window: window.open(); // In child window: console.log(window.opener); // outputs a Window object // Now, click the X on the parent window. // Continue below in child window: console.log(window.opener); // outputs null // However, you can still close the window! window.close(); // closes the child window ``` On the other hand, if the user loaded a page with this code in a new tab: ``` console.log(window.opener); // also outputs null window.close(); // doesn't work; window remains open ``` Firefox then complains in the Error Console about a script not being able to close windows not opened by scripts, and Chrome does nothing. The reason that I need to check whether `window.close()` will close the window is that if the window remains open, I want to go to another address. I thought of doing it this way: ``` // Try to close window. window.close(); // If the window is still open after three seconds, go to another page. setTimeout(function(){ // For testing purposes: alert("Not closed!"); // What I want to do: window.location = "http://www.example.com/"; }, 3000); ``` However, a three-second lag would make my application seem slow to the user. That would not do. On my computer, a delay of 1 millisecond was sufficient to let the window close; the alert only happened if the window remained open. However, I would need someone to confirm that this is true for all computers. This also does not work: ``` try{ // This will not throw an error regardless of // whether the window was actually closed: window.close(); }catch(e){ // This never happens: console.log("Could not close window"); } ``` In short, I just need a way in JavaScript to know, either before or after calling `window.close()`, whether the window will actually close. How do I do that?