Prevent a User Navigating Away from a Page with Unsaved Changes (http://www.jacopretorius.net/2013/12/prevent-a-user-navigating-away-from-a-page-with-unsaved-changes.html)

gistfile1.js

$.fn.extend({
  trackChanges: function() {
    $(":input", this).keyup(function() {
      $(this.form).data("changed", true);
    });
 
    var form = $(this);
    $(form).submit(function() {
      form.data("submitted", true);
    });
  },
  isChanged: function() {
    return this.data("changed") && !this.data("submitted");
  }
});
 
$(document).ready(function() {
  if ($('.detect_changes').length > 0) {
    $('.detect_changes').trackChanges();
    $(window).on('beforeunload', function() {
      if ($('.detect_changes').isChanged()) {
        return 'Your changes have not been saved.';
      }
    });
  }
});