实例

                var bodyH = document.body.clientHeight
        var vh = document.documentElement.clientHeight
        console.log(vh);
        console.log(bodyH);
        // 滚动条滚动的距离+vh ==body
        window.onscroll = function(){
            var scrollTop = window.scrollY
            console.log(scrollTop);
        }
        // scrollTop + vh == bodyH      判断页面到底部了