Capturing window.onbeforeunload

The reason why nothing happens when you use 'alert()' is probably as explained by MDN: "The HTML specification states that calls to window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event."

But there is also another reason why you might not see the warning at all, whether it calls alert() or not, also explained on the same site:

"... browsers may not display prompts created in beforeunload event handlers unless the page has been interacted with"

That is what I see with current versions of Chrome and FireFox. I open my page which has beforeunload handler set up with this code:

window.addEventListener
('beforeunload'
, function (evt)
  { evt.preventDefault();
    evt.returnValue = 'Hello';
    return "hello 2222"
  }
 );

If I do not click on my page, in other words "do not interact" with it, and click the close-button, the window closes without warning.

But if I click on the page before trying to close the window or tab, I DO get the warning, and can cancel the closing of the window.

So these browsers are "smart" (and user-friendly) in that if you have not done anything with the page, it can not have any user-input that would need saving, so they will close the window without any warnings.

Consider that without this feature any site might selfishly ask you: "Do you really want to leave our site?", when you have already clearly indicated your intention to leave their site.

SEE: https://developer.mozilla.org/en-US/docs/Web/Events/beforeunload

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;
});

There seems to be a lot of misinformation about how to use this event going around (even in upvoted answers on this page).

The onbeforeunload event API is supplied by the browser for a specific purpose: The only thing you can do that's worth doing in this method is to return a string which the browser will then prompt to the user to indicate to them that action should be taken before they navigate away from the page. You CANNOT prevent them from navigating away from a page (imagine what a nightmare that would be for the end user).

Because browsers use a confirm prompt to show the user the string you returned from your event listener, you can't do anything else in the method either (like perform an ajax request).

In an application I wrote, I want to prompt the user to let them know they have unsaved changes before they leave the page. The browser prompts them with the message and, after that, it's out of my hands, the user can choose to stay or leave, but you no longer have control of the application at that point.

An example of how I use it (pseudo code):

onbeforeunload = function() {

  if(Application.hasUnsavedChanges()) {
    return 'You have unsaved changes. Please save them before leaving this page';
  }


};

If (and only if) the application has unsaved changes, then the browser prompts the user to either ignore my message (and leave the page anyway) or to not leave the page. If they choose to leave the page anyway, too bad, there's nothing you can do (nor should be able to do) about it.