Attach DOMContentLoaded listener to dynamically created window

Question

Due to how some of our pages work, JS can get injected into the page at any point and sometimes this JS closes the current window. The problem is that I need to attach an event listener to the onunload of the window so that a value may be returned from the window to the parent page. But because the window close script may be injected at any point, I can't bind this event to the onload due to how it works so I was hoping to use DOMContentLoaded since that event will trigger before the injected script does.

However in my tests, I cannot get anything to bind to DOMContentLoaded on the parent page where the new window is being created.

Here is an what I am currently working with: Plunker

We only need this to work in Chrome at the moment.

Our current method of doing this works like this (pseudocode):

onButtonClick = function(){
    win = window.open(...);
    win.onload = function(){
        win.onunload = function(){
            //Bind some function that will get the window's "return value" and pass it to the parent page
            //This will never happen if the window closes itself before the page is done loading
        };
    };
};

Can I use DOMContentLoaded to accomplish what I want? If so, how do I properly attach it to the window?

Note: I cannot bind the "onunload" event directly to the window once it is created. It seems to fire the "onunload" event twice (once when the window opens and once when it closes). You can see this happening if you use the "bindOnCreate" function in my example.

Answer 1

If you change line 58 from

w.document.addEventListener('DOMContentLoaded', ...)

to

w.addEventListener('DOMContentLoaded', ...)

it works. I'll try to explain what's actually going on under the hood:

• When window is opened, it is initially having URL about:blank. You can check this by logging w.location.toString() in onunload event handler (see next step).
• Immediately after that the browser loads URL supplied in window.open, thus triggering onunload for about:blank (first time).
• Real page with different window.document is loaded into pop-up window, but your event handlers are still listening to the DOM root of about:blank page because you added events to window.document, not window; and right now as we have another URL loaded, window.document is completely different object than one step before.
• When you close window, onunload is triggered again (second time) because your onunload event was connected to window.

If you addEventListener for pop-up's window, it receives events from all window.document-s that will be loaded inside that window because of JS event bubbling mechanism.

I hope this answers your questions.