var sidebar = document.getElementById("guding");
var sidebarPos = sidebar.offsetTop;

window.onscroll = function() {
if (window.pageYOffset >= sidebarPos) {
  sidebar.style.position = "fixed";
  sidebar.style.top = "0";
} else {
  sidebar.style.position = "relative";
  sidebar.style.top = "auto";
}

// 判斷側邊欄是否已經貼合頂部
if (window.pageYOffset <= sidebarPos) {
  sidebar.style.position = "relative";
  sidebar.style.top = "0";
}
};