Question

I want to execute a JavaScript on unload of a child window which was opened by the parent window. I tried the below code and it is not calling the function.

 childWindow = window.open(url, 'MyWindow', GetWindowOptions(1020, 600), true);
 childWindow.onunload = function () { test(); };

And the test function which I wrote is:

function test() {
alert(1);
}

I am using IE8 browser.

Answer 1

Replace attachEvent instead of the onunload setter to add the event. I’ve tested it in IE6 – 8, and it works fine. Make sure that you also use addEventListener for IE9+ and other browsers:

var unloadFunc = function () { test(); };
if (childWindow) { // null if a pop-up blocker does not create the window
    if (childWindow.addEventListener) {
        childWindow.addEventListener('unload', unloadFunc, false);
    } else {
        childWindow.attachEvent('onunload', unloadFunc);
    }
}

Obviously, this will not work if the URL is from a different domain.
If you want to execute a function when a window from a different origin is closed, use setInterval or setTimeout to poll for the value of the boolean property childWindow.closed. It’s true when the window has been closed.

For example:

if (childWindow) { // null if a pop-up blocker does not create the window
    setTimeout(function checkState() {
        if (childWindow.closed) {
            // onunload Logic here.
        } else {
            setTimeout(checkState, 250);
        }
    }, 250);
}

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Note: Other answers suggested to use the beforeunload event. Keep in mind that the specification allows implementations to ignore confirm, alert, prompt and showModalDialog calls during this event.