Example usage of onbeforeunload(). Used extensively in GMail, Google Docs, CodePen etc when there are unsaved changes.

gistfile1.js

// Pseudo code for using onbeforeunload()
$(window).on('unsaved', function(event) {
  window.onbeforeunload = function() {
    return "SOMETHING BAD if you navigate away.";
  }

}).on("save", function(event) {
  window.onbeforeunload = void(0);
});