how to get hidden value in javescript

Question

 <script type="text/javascript">


           
function uploadStarted() {
    $get("imgDisplay").style.display = "none";
}
function uploadComplete(sender, args) {
    var imgDisplay = $get("imgDisplay");
    imgDisplay.src = "images/loader.gif";
    imgDisplay.style.cssText = "";
    var img = new Image();
    img.onload = function () {
        imgDisplay.style.cssText = "height:150px;width:150px";
        imgDisplay.src = img.src;
    };
    img.src = "<%=ResolveUrl(filename) %>" // "<%=ResolveUrl(hdnImage.ClientID) %>"; // args.get_fileName();
}
</script>


<%=ResolveUrl(hdnImage.ClientID) %> : hdnImage is hiddenvalue which contain the file name 


Question:I am not able to get the filename to the hdnImage 
how can i get it in the javascript..pls help..!!