如何获取页面中按钮控件距离头部的距离啊？

function  getScrollTop () {
            var scrollPos;
            if (window.pageYOffset) {
                scrollPos = window.pageYOffset;
            }
            else if (document.compatMode && document.compatMode != 'BackCompat') {
                scrollPos = document.documentElement.scrollTop;
            }
            else if (document.body) {
                scrollPos = document.body.scrollTop;
            }
            return scrollPos;
        }