Why javascript is not setting asp.net hidden field value ?

I am using javascript to set

asp:hiddenfield

 to '1' but not getting set.

I am setting it like this:

<script type="text/javascript">
function uploadComplete(sender, args) {
var myHidden = document.getElementById('<%= HdnFieldEmployeePicture.ClientID %>');
myHidden.value = '1';
}</script>

from:

<asp:AsyncFileUpload ID="FileUpload1" OnClientUploadComplete="uploadComplete" ClientIDMode="AutoID" UploaderStyle="Modern" runat="server"/> <asp:HiddenField ClientIDMode="Static" ID="HdnFieldHasFileUploaded" runat="server" />

I am checking it on server side:

if (HdnFieldHasFileUploaded.Value == "1")
{

but not set to 1.

AsyncControl and hidden field are inside UpdatePanel.