var x = 2;
function add() {
    x = x + 2;
}

var scroll = $('.verticalScrollBar');
function changeScrollBar() {
    ...
    TweenMax.set(scroll, {css:{height:scrollBarHeight+'%'}});
    ...
}

function changeScrollBar() {
    ...
    TweenMax.set($('.verticalScrollBar'), {css:{height:scrollBarHeight+'%'}});
    ...
}