js 窗口滚动到一定高度时加载数据

<script type="text/javascript">
        //当窗口滚动到一定高度时 某块页面开始加载数据
        window.onload = function()
        {
            var oHeader = document.getElementById("header");
            var height = oHeader.offsetHeight;
            
            window.onscroll = function(){
                //scrollTop的值
                var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
                //窗口可视化高度
                var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;
                document.getElementById("txt1").value="scrollTop:"+scrollTop+",height:"+height;
                var oContent = document.getElementById("content");
                
                if(scrollTop+clientHeight >height)
                {
                   oContent.innerHTML = "<h1>正在加载中.....</h1>";
                   
                   //延迟3秒执行
                   setTimeout(function(){
                     oContent.innerHTML = "<h1>scroll事件执行的代码块</h1>";
                     oContent.style.background="red";
                   },3000);  
                  
                }
                else
                {
                    oContent.innerHTML="";
                    oContent.removeAttribute("style");
                }
            }
        }
    </script>