<html>
<head>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"></script>
<script type="text/javascript" language="javascript">
function change() {
if ($('#aa:visible').length > 0)
$('#aa').hide();
else
$('#aa').show();
}
</script>
</head>
<body>
<div id="aa">blablabla</div>
<input type="button" onclick="change();" value="change" />
</body>
</html>