I use this snippet to prevent the user from accidentally navigating away from the editing page:

var warning = false;
window.onbeforeunload = function() {
  if (warning) {
    return '';
  }
}

Now I want to set the variable warning to be true if any of the textarea or input type="text" has been modified, I want to use the onkeyup event such as in:

document.getElementById('textarea_id').onkeyup = function() {
    warning = true;
}

But it's not working. Don't quite want to use jQuery on this. Any insights?