Set value of ASP.Net Hidden Field using JavaScript

<asp:HiddenField ID = "HiddenField1" runat = "server" />
<script type = "text/javascript">
    window.onload = function () {
        document.getElementById("<%=HiddenField1.ClientID %>").value = "Test";
        alert(document.getElementById("<%=HiddenField1.ClientID %>").value);
    };
</script>