Call HttpHandler from javascript

I have a simple page with button which calls HttpHandler via JavaScript.

HttpHandler gets lots of files and adds them to a zip file, after finishing work zip file will be added to Response.

This operation can take several minutes. I would like to execute some JavaScript function after finishing work of HttpHandler.

How can I do it?

My code:

<asp:Button ID="btnDownload" runat=server Text="Download" OnClientClick="Download()" />

<script type="text/javascript">
    function Download()
    {
        var url = 'F开发者_开发百科ileStorage.ashx';
        window.open(url);            
    }
</script>

UPD 1:

I have found other solution. Using XMLHttpRequest.

Code:

 <script type="text/javascript">
        var xmlHttpReq = createXMLHttpRequest();

        function Download() {
            var url = 'FileStorage.ashx';            
        xmlHttpReq.open("GET", url, false);
        xmlHttpReq.onreadystatechange = onResponse;
        xmlHttpReq.send(null);         
    }

    function onResponse() {
        if (xmlHttpReq.readyState != 4)
        { return; }        
        var serverResponse = xmlHttpReq.responseText;
        alert(serverResponse);        
     }

    function createXMLHttpRequest() {
        try { return new XMLHttpRequest(); } catch (e) { }
        try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) { }
        try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch (e) { }
        alert("XMLHttpRequest not supported");
        return null;
    }
    </script>

In onResponse() I can see my zip file in responseText (binary). But I don't have any idea how I can say to browser to download result of working httphandler such as file.

Any ideas?

---

I would use JQuery AJAX and then on success, write a function that will do whatever work you need it to. Plus with AJAX you can show the user an icon that says loading so they know something is actually processing, instead of the page just hanging and nothing appearing to happen.

$.ajax({
  url: "FileStorage.ashx",
  context: document.body,
  success: function(){
     // Whatever you want to do here in javascript.
  }
});