（急）求助，焦点图JS报错

<SCRIPT language=javascript type=text/javascript>
var currslid = 0;
var slidint;
function setfoc(id)
{document.getElementById("focpic").src = picarry[id];
 document.getElementById("foclnk").href = lnkarry[id];
 document.getElementById("fttltxt").innerHTML = "<a href=\""+lnkarry[id]+"\" target=_blank>"+ttlarry[id]+"</a>";
 currslid = id;
 for(i=0;i<3;i++)
  {document.getElementById("tmb"+i).className = "thubpic";};
 document.getElementById("tmb"+id).className ="thubpiccur";
 focpic.style.visibility = "hidden";
 focpic.filters[0].Apply();
 if (focpic.style.visibility == "visible")
{focpic.style.visibility = "hidden";
 focpic.filters.revealTrans.transition=12;}
 else
{focpic.style.visibility = "visible";
 focpic.filters[0].transition=12;}
 focpic.filters[0].Play();
 stopit();}
function playnext()
{if(currslid==3)
{currslid = 0;}
 else
  {currslid++;};
 setfoc(currslid);
 playit();}
function playit()
{slidint = setTimeout(playnext,4500);}
function stopit()
{clearTimeout(slidint);}
window.onload = function(){playit();}
</SCRIPT>
这是我的JS，焦点图切换，第三张图片返回到第一张图片的时候会有报错
求教原因，我JS小白，还请各位大大指教！