var ModalHelper = (function(bodyCls) {
                var scrollTop;
                return {
                afterOpen: function() {
                  scrollTop = document.scrollingElement.scrollTop;
                  document.body.classList.add(bodyCls);
                  document.body.style.top = -scrollTop + 'px';
                },
                beforeClose: function() {
                  document.body.classList.remove(bodyCls);
                  // scrollTop lost after set position:fixed, restore it back.
                  document.scrollingElement.scrollTop = scrollTop;
                }
              };
})('modal-open');