//滾動條在Y軸上的滾動距離
function getScrollTop() {
    var scrollTop = 0,
      bodyScrollTop = 0,
      documentScrollTop = 0;
    //兼容谷歌
    if (document.body) {     bodyScrollTop = document.body.scrollTop;   }
    //兼容火狐
    if (document.documentElement) {     documentScrollTop = document.documentElement.scrollTop;   }
       scrollTop = (bodyScrollTop - documentScrollTop > 0) ? bodyScrollTop : documentScrollTop;
    return scrollTop;
}
//文檔的總高度
function getScrollHeight() {
    var scrollHeight = 0,
      bodyScrollHeight = 0,
      documentScrollHeight = 0;
    //兼容谷歌
    if (document.body) {
      bodyScrollHeight = document.body.scrollHeight;
    }
    //兼容火狐
    if (document.documentElement) {
      documentScrollHeight = document.documentElement.scrollHeight;
    }
    scrollHeight = (bodyScrollHeight - documentScrollHeight > 0) ? bodyScrollHeight : documentScrollHeight;
    return scrollHeight;
}
//瀏覽器視口的高度
function getWindowHeight() {
    var windowHeight = 0;
    windowHeight = document.documentElement.clientHeight;
    return windowHeight;
}
window.onscroll = function() {
    if (getScrollTop() + getWindowHeight() == getScrollHeight()) {
      alert("you are in the bottom!");
    }
};

$(window).scroll(function(){
  var scrollTop = $(this).scrollTop();
  var scrollHeight = $(document).height();
  var windowHeight = $(this).height();
  if(scrollTop + windowHeight == scrollHeight){
    alert("you are in the bottom");
  }
});