The MDN recommended way of executing unload code

unload.js

// https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon#sending_analytics_at_the_end_of_a_session

const callbacks = [];

function unload(callback) {
    callbacks.push(callback);
};

document.addEventListener('visibilitychange', () => {
    if (document.visibilityState === 'hidden') {
        callbacks.forEach((callback) => callback());
    }
});

// Usage:

unload(() => {
    navigator.sendBeacon('/log', analyticsData);
});