把一个类名赋给一个元素，提示类名未定义

Question

问题如题，代码如下：
function formEvent(elem,cn){
var element=document.getElementById(elem);
var old_class=element.getAttribute("class");
elem.onmouseover=function(){
this.className=cn;
}
elem.onmouseout=function(){
this.className=old_class;
}
}
function message(){
var submitbtn=document.getElementById("submitbtn");
formEvent(submitbtn,submit1over);
}

Answer 1

<style>
.a1{background:#ccc}
.submit1over{background:#000}
</style>
<script>
function formEvent(elem,cn){ 
var element = document.getElementById(elem); 
var old_class = element.className;
element.onmouseover=function(){ 
this.className=cn; 
} 
element.onmouseout=function(){
this.className=old_class; 
} 
} 
function message(){ 
formEvent("submitbtn","submit1over"); 
}
window.onload = function(){
message()
}
</script>
<div id="submitbtn" class="a1">xxxxxxxxx</div>