//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");
            }
        }