You can use onunload event attached to the opened window;

var w = open('stackoverflow.com');
w.onunload = (ev) => {
    console.log('unload', ev);
};
//w.close();

Update

Set unload even inside load event to prevent from unnecessary firing unload on window open, witch happen to me in chrome at least.

var w = open('https://stackoverflow.com/users/8424614/unnamedxaer');
w.onload = () => {
    w.onunload = (ev) => {
        console.log('unload', ev);
    };
};

Update 2

The urls have to be in the same domain to work, otherwise you will get an error when trying to attach event to opened window (w) (set debugger in the line with onunload), they also must be served from same kind of server - not accessed like files when the url in browser looks like this c:/my-site/welcome.php.

Here is working example. Open CodeSandbox's console located at the bottom of the window below the browser section to see output.