光棒效果求指教，不知道哪里出错误了

<script type="text/javascript">
var trs=document.getElementsByName("tr");
var bgc;
var c;
for(var i=1;i<trs.length;i++){
if(i%2===0){
trs[i].style.backgroundColor="yellow"
trs[i].style.color="red";
}
trs[i].onmouseover=function(){
bgc=this.style.backgroundColor;
c=this.style.color;
this.style.backgroundColor="blue";
this.style.color="white";
alert(this);
};
trs[i].onmouseout=function(){
this.style.backgroundColor=bgc;
this.style.color=c;
};
}
</script>