求，提示这个是我哪里错了呀？怎么改？Uncaught TypeError: Cannot set property 'onmouseover' of null
<script type="text/javascript">
window.onload = function(){
var chgOpacity=document.getElementById('chgOpacity');
oShareSide.onmouseover = function (){
startMove(0);
}
oShareSide,onmouseout = function(){
startMove(-200);
}
}
var chgOpacity=document.getElementById('chgOpacity');
chgOpacity.onmouseover = function (){
startChgOpac(100);
}
chgOpacity.onmouseout = function(){
startChgOpac(30);
}
var doing = null;
var alpha = 30;
function startChgOpac(iTarget){
clearInterval(doing);
doing = setInterval(function(){
var speeds = 0;
if (alpha > iTarget) {
speeds = -10 ;
}else{
speeds = 10;
}
if(alpha == iTarget){
clearInterval(doing);
}else{
alpha += speeds;
chgOpacity.style.filter='alpha(opacity='+alpha+')';
chgOpacity.style.opacity = alpha/100;
}
},30);
}
