Capturing window.onbeforeunload

You have to return from the onbeforeunload:

window.onbeforeunload = function() {
    saveFormData();
    return null;
}

function saveFormData() {
    console.log('saved');
}

UPDATE

as per comments, alert does not seem to be working on newer versions anymore, anything else goes 🙂

FROM MDN

Since 25 May 2011, the HTML5 specification states that calls to window.showModalDialog(), window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event.

It is also suggested to use this through the addEventListener interface:

You can and should handle this event through window.addEventListener() and the beforeunload event.

The updated code will now look like this:

window.addEventListener("beforeunload", function (e) {
  saveFormData();

  (e || window.event).returnValue = null;
  return null;
});