Try this
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
var a = "City1";
function Toggle() {
document.getElementById("div1").style.display = a == "City" ? "block" : "none";
}
</script>
</head>
<body>
<form id="form1" runat="server">
<input type = "button" value = "Toggle" onclick = "Toggle()" />
<div id = "div1">DIV1</div>
</form>
</body>
</html>