Remove the on from onbeforeunload.

Also, be aware that addEventListener will not work in the older IE's and possibly other browsers. If you want consistent event binding use a library.

There is an "almost cross-browser working example" at Mozila Developer Network API reference for beforeunload event. Use their code.

in 2014, that was

window.addEventListener("beforeunload", function (e) {  var confirmationMessage = "\o/";  (e || window.event).returnValue = confirmationMessage;     //Gecko + IE  return confirmationMessage;                                //Webkit, Safari, Chrome etc.});

in 2020, it now is

window.addEventListener('beforeunload', (event) => {  // Cancel the event as stated by the standard.  event.preventDefault();  // Chrome requires returnValue to be set.  event.returnValue = '';});

all of the above?

If I ever needed this, I'd want to entrust the job to a library. If I had to do this myself, I imagine one can do all of the above, just to be extra sure

• do not try to set meaningful message text, it will only give inconsistent UX
• event = event || window.event
• event.preventDefault(), maybe after check that preventDefault is defined?
• event.returnValue = ''
• return ''

There's no prefix on for the EventListeners but it's applicable or may I say necessary for EventHandlers

So, just keep in mind that

EventHandlers = prefix on

EventListeners = prefix off