Example code for window.onbeforeunload (not generally usable!) There are two event-handlers for loggedOut and loggedIn.

gistfile1.js

    // App.logout is just a AJAX call doing logout
    var App = {
        
        // ...
        
        logout: function () {
            return $.ajax({
                url: REST.postLogout,
                type: 'POST'
            });
        }
    }

    App.when = {
        loggedOut: function () {
            $('html').removeClass('logged-in').addClass('logged-out');
            window.onbeforeunload = null;
        },

        loggedIn: function () {
            $('html').removeClass('logged-out').addClass('logged-in');
            window.onbeforeunload = function () {
                App.logout();
                // return nothing, otherwise a confirm dialog of the browser appears
            };
        }
    };