有关动态生成带事件的表格

for(var y=0;y<data.roms.length;y++)
{
var _tr=document.createElement("tr"); 
var _td1=document.createElement("td"); 
_td1.innerHTML =data.roms[y].roomno+'-房间'+data.roms[y].roomid;
//_td1.innertitle=data.roms[y].roomid;
//_tr.onclick=function(){findHouseWareByHouseId(y);};//{findHouseWareByHouseId(data.roms[y].roomno)};
_tr.appendChild(_td1); 

_tr.attachEvent('onclick',function(){alert(data.roms[y].roomid);});
//_tr.setAttribute('ondbclick',findHouseWareByHouseId(12));
sonromtab.appendChild(_tr); 
}
上述代码中为什么_tr.attachEvent('onclick',function(){alert(data.roms[y].roomid);});里面alert的值取不到