Fixed side vertical Menu with limited scrolling

How do I make a menu that does not even Wordpress that is vertical, fixed and has its own scroll that is limited to its ends:

var last_scroll;

In wordpress the element div receives a "position:fixed" when reaching the end and the beginning of the menu to stop interacting with the scroll, and when scrolling it defide "position:absolute" and "top: Numeroquenaoconsigofazar".

I was trying to do with javascript but it's Impossible:

$(window).scroll(function(event) {
    element = $('#sidebar_main');
    p_current = $(this).scrollTop();
    h_nav = element.height();
    h_screen = $(window).height();

    if(h_nav > h_screen){//se maior que a tela
        h = (h_nav - h_screen)+40;

        if(p_current > last_scroll){//se desce
            if(p_current>=h){//se já chegou no fim
                element.css({
                    'position': 'fixed',
                    'bottom': '0',
                    'top': 'auto'
                });
            }else{
                if(element.css('top') == 'auto'){
                    element.css('top', 0);
                }
                element.css({
                    'top': '+=1px',
                    'position': 'absolute',
                    'bottom': 'auto'
                });
            }
        }else{//se sobe

            if(p_current<=0){//se já chegou no começo
                element.css({
                    'top': '0',
                    'bottom': 'auto'
                });
            }else{
                //if(element.offset().top <= h || element.css('top') == 'auto'){


                    if(element.css('top') == 'auto'){
                        element.css('top', element.offset().top);
                    }
                    element.css({
                        'top': '-=1px',
                        'position': 'absolute',
                        'bottom': 'auto'
                    });
                }
            //}
        }

        last_scroll = p_current;
    }

});