js无缝滚动

var step=5;
		var speed=200;
		var demo1=document.getElementById("demo1");
		var demo2=document.getElementById("demo2");
		var demo3=document.getElementById("demo3"); 
		demo3.innerHTML=demo2.innerHTML;
		function autorun()
		{
			if(demo2.offsetHeight-demo1.scrollTop>=0)
			{
				demo1.scrollTop+=step;
			}else
			{
				demo1.scrollTop=0;	
			}
			
		}
		setInterval("autorun()",speed);