is it possible to open a popup with javascript and then detect when the user closes it?

If you have control over the contents of the pop-up, handle the window's unload event there and notify the original window via the opener property, checking first whether the opener has been closed. Note this won't always work in Opera.

window.onunload = function() {
    var win = window.opener;
    if (!win.closed) {
        win.someFunctionToCallWhenPopUpCloses();
    }
};

Since the unload event will fire whenever the user navigates away from the page in the pop-up and not just when the window is closed, you should check that the pop-up has actually closed in someFunctionToCallWhenPopUpCloses:

var popUp = window.open("popup.html", "thePopUp", "");
function someFunctionToCallWhenPopUpCloses() {
    window.setTimeout(function() {
        if (popUp.closed) {
            alert("Pop-up definitely closed");
        }
    }, 1);
}

If you don't have control over the contents of the pop-up, or if one of your target browsers does not support the unload event, you're reduced to some kind of polling solution in the main window. Adjust interval to suit.

var win = window.open("popup.html", "thePopUp", "");
var pollTimer = window.setInterval(function() {
    if (win.closed !== false) { // !== is required for compatibility with Opera
        window.clearInterval(pollTimer);
        someFunctionToCallWhenPopUpCloses();
    }
}, 200);

There is a very simple solution to your problem.

First make a new object which will open up a pop like this :

var winObj = window.open('http://www.google.com','google','width=800,height=600,status=0,toolbar=0');

In order to know when this popup window is closed, you just have to keep checking this with a loop like the following :

var loop = setInterval(function() {   
    if(winObj.closed) {  
        clearInterval(loop);  
        alert('closed');  
    }  
}, 1000); 

Now you can replace alert with any javascript code you want.

Have Fun! :)

Try looking into the unload and beforeunload window events. Monitoring these should give you an opportunity to call back when the DOM unloads when the window is closed via something like this:

var newWin = window.open('/some/url');
newWin.onunload = function(){
  // DOM unloaded, so the window is likely closed.
}