id加不上~~alert没反应~~求大神帮忙看一下~~~

Question

window.onload=function(){         var titles=document.getElementById("month_tit").getElementsByTagName("li");         var divs=document.getElementById("month_con").getElementsByTagName("div");         if(titles.length!=divs.length)                 return;         for(var i=0;i<titles.length;i++){                 titles[i].index=i;                 titles[i].onmouseover=function(){                         for(var j=0;j<titles.length;j++){                            titles[j].className="";                            divs[j].style.display="none";                         }                         this.className="selected";                         divs[this.id].style.display="block";                 }         } }