取得 body 捲動後的高度

var bodyTop = 0;
if (typeof window.pageYOffset != 'undefined') {
  bodyTop = window.pageYOffset;

} else if (typeof document.compatMode != 'undefined' && document.compatMode != 'BackCompat') {
  bodyTop = document.documentElement.scrollTop;

} else if (typeof document.body != 'undefined') {
  bodyTop = document.body.scrollTop;
}