Call server side function before unload (firebase)

March 16, 2022

I’m trying to save user data when the JavaScript event "beforeunload" is triggered. I call a server-side function (firebase cloud functions) that simply gets an object from the user and updates a document. The function does trigger, but from the firebase console I can see it finished with code 204.
Here is my client code

window.onbeforeunload = function() {
    firefun_ref(data);
}

Server side:

exports.updateStats = functions.https.onCall(async (data, context) => {
  return admin.firestore().collection("stats")
    .doc((new Date()).toISOString().substring(0, 10))
    .update(filter(data));
});

I want to point out that if I run the function by itself it works fine, it just gives problems when it’s in the before unload event

>Solution :

This will not be possible.

Browsers severely limit what you’re allowed to do in a beforeunload event, because it’s so prone to abuse. Asynchronous events are not allowed (because they could force the window to stay open for an arbitrary amount of time after the user wants to close it.)

As far as I know, the only reliable usage of onbeforeunload is to display a prompt allowing the user to change their mind; some browsers will let you customize the text that appears (by returning a string from the beforeunload handler), some browsers will always show default text:

window.addEventListener('beforeunload', function (e) {
  // Cancel the event
  e.preventDefault(); // If you prevent default behavior in Mozilla Firefox prompt will always be shown
  // Chrome requires returnValue to be set
  e.returnValue = '';
});

https://developer.mozilla.org/en-US/docs/Web/API/WindowEventHandlers/onbeforeunload