如何取多选框的值,并构造成"aaa,bbb,ccc"这样的字符串?

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title> New Document </title>
<meta name="Generator" content="EditPlus">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
<script language="JavaScript">
<!--
function sel()
{
var o = document.all.se.options;
var t = "";
for(var i=0;i<o.length;i++)
{
if(o[i].selected)
{
t += o[i].text + ",";
}
} if(t.length>0)
{
return t.substr(0,t.length-1);
}
else
{
return "";
}
}
//-->
</script>
</head><body>
<select name="se" size=6 multiple> 
<option>aaa</option>
<option>bbb</option>
<option>ccc</option>
<option>ddd</option>
</select><input type="button" onclick="alert(sel())" value="getselect">
</body>
</html>