beforeunload on IE 11 - do not prompt user does not work

Question

When adding a listener to the global window object for the beforeunload event, IE 11 (and 10) does not behave as Chrome and Firefox.

Normally, you return a

Answer 1

The solution is not to return anything (which is the same as return; or return undefined;).

var isDirty = false;
var message = '** You have unsaved changes. **'
window.addEventListener('beforeunload', function(evt){
  if(isDirty) {
    evt.returnValue = message;
    return message;
  }
  delete evt.returnValue;
});