jQuery UI Dialog OnBeforeUnload

The correct way to display the alert is to simply return a string. Don't call the alert() method yourself.

<script type="text/javascript">
    $(window).on('beforeunload', function() {
        if (iWantTo) {
            return 'you are an idiot!';
        }
    }); 
</script>

See also: https://developer.mozilla.org/en-US/docs/Web/Events/beforeunload

You can also make an exception for leaving the page via submitting a particular form:

$(window).bind('beforeunload', function(){
    return "Do you really want to leave now?";
});

$("#form_id").submit(function(){
    $(window).unbind("beforeunload");
});

this works for me

$(window).bind('beforeunload', function() {
      return 'Do you really want to leave?' ;
});

jQuery API specifically says not to bind to beforeunload, and instead should bind directly to the window.onbeforeunload, I just ran across a pretty bad memory in part due binding to beforeunload with jQuery.

This works for me:

window.addEventListener("beforeunload", function(event) {
  event.returnValue = "You may have unsaved Data";
});