//滚动条距离顶部高度

        function getScrollTop() {

            var scrollTop=0;

            if(document.documentElement&&document.documentElement.scrollTop) {

                scrollTop=document.documentElement.scrollTop;

            }

            else if(document.body)  {

                scrollTop= document.body.scrollTop;

            }

            return  Math.ceil(scrollTop);

        }

        //滚动条本身高度：就是可视窗口高度

        function getScrollBarHeight(){

            let scrollBarHeight = document.documentElement.clientHeight;

            return Math.ceil(scrollBarHeight);

        }

        //整个页面高度

        function getPageHeight()  {

            return Math.ceil(Math.max(document.body.clientHeight,document.documentElement.scrollHeight));

        }

        window.onscroll = function () {

            let top = getScrollTop();

            let ch = getScrollBarHeight();

            let sh = getPageHeight();

            if (top + ch >= sh) {

                console.log("到达底部");

            }else{

               console.log("没有到达底部");

            }

        }