Is it possible to display a custom message in the beforeunload popup?

When using window.onbeforeunload (or $(window).on("beforeonload")), is it possible to display a custom message in that popup?

Not anymore. All major browsers have started ignoring the actual message and just showing their own.

By looking at existing answers I have the feeling this was possible in the past using things like confirm or alert or event.returnValue, but now it seems they are not working anymore.

Correct. A long time ago, you could use confirm or alert, more recently you could return a string from an onbeforeunload handler and that string would be displayed. Now, the content of the string is ignored and it's treated as a flag.

When using jQuery's on, you do indeed have to use returnValue on the original event:

$(window).on("beforeunload", function(e) {
    // Your message won't get displayed by modern browsers; the browser's built-in
    // one will be instead. But for older browsers, best to include an actual
    // message instead of just "x" or similar.
    return e.originalEvent.returnValue = "Your message here";
});

or the old-fasioned way:

window.onbeforeunload = function() {
    return "Your message here"; // Probably won't be shown, see note above
};

That's all you can do.

tl;dr - You can't set custom message anymore in most modern browsers

A quick note (since this is an old answer) - these days all major browsers don't support custom message in the beforeunload popup. There is no new way to do this. In case you still do need to support old browsers - you can find the information below.

In order to set a confirmation message before the user is closing the window you can use

jQuery

$(window).bind("beforeunload",function(event) {
    return "You have some unsaved changes";
});

Javascript

window.onbeforeunload = function() {
    return "Leaving this page will reset the wizard";
};

      It's important to notice that you can't put confirm/alert inside beforeunload

A few more notes:

1. NOT all browsers support this (more info in the Browser compatibility section on MDN) 2. In Firefox you MUST do some real interaction with the page in order for this message to appear to the user.
   3. Each browser can add his own text to your message.

Here are the results using the browsers I have access to:

Chrome:

Firefox:

Safari:

IE:

Just to make sure - you need to have jquery included

More information regarding the browsers support and the removal of the custom message:

1. Chrome removed support for custom message in ver 51
2. Opera removed support for custom message in ver 38
3. Firefox removed support for custom message in ver 44.0 (still looking for source for this information)
4. Safari removed support for custom message in ver 9.1