这么写为什么不行

<script type=text/javascript>

window.onload=function(){

var bo1=document.getElementById(box);

bo1.onmouseover=function(){

boc(1);

}

bo1.onmouseout=function(){

boc(0.3);}

}

function boc(ita)

{

var bo1=document.getElementById(box);

    bo1.style.opacity=ita;

}

</script>