Forgive for a moment that there are vastly better ways to handle a simple toggle like this and pretend that we have to do it by declaring inline styles.

What's wrong with my logic such that I can't call toggleSidebar(); every time a button a clicked and always produce the opposite result of what is currently true? (This only works the first time you click the button right now).

function toggleSidebar() {
    var sidebar = document.getElementById('sidebar');
    var show = Boolean;
    
    if (show == null) {
        show = true;
    }

    if (show) { 
        sidebar.style.right = '0px';
        show = !show;
    } else {    
        sidebar.style.right = '-400px';
        show = !show;
    }
}