How to generate alert or js function after generic handler

I have written a function which basically takes some input as text and set of files. And using Generic handler i save the files and text all works fine. The issue is after the process i want to some some alert or success message how can i achieve that?

JS:

 var appID = $("#hidden_applicationID").val();
        var messageText = escape($("#screeningFeedback").val());

        var files = $("#fileScreening").get(0).files;
        var fileData = new FormData();

        for (var i = 0; i < files.length; i++) {
            fileData.append("fileInput", files[i]);
        }

        fileData.append("uniqueapplicationID", appID);
        fileData.append("messageText", messageText);

         $.ajax({
            url: "ScreeningUpload.ashx",
            data: fileData, type: "POST",
            processData: false,
            contentType: false,
            cache: false,
            dataType: "json",
             success: function (mydata) {
                 alert("hello");
                //$("#screeningFeedback").val();
                //showtoaster("Thank you for your submission!");

            }

        });

C#

 public void ProcessRequest(HttpContext context)
{ // DOING SOMETHING


    context.Response.Clear();
    context.Response.Write("<script type='text/javascript'>alert('Hello, world');</script>");

 }

How can i achieve the above tried many things but no luck.

Thank you

If you are sending JavaScrip as response from backend then add that response in DOM. Browser will execute it.

$.ajax({
        url: "ScreeningUpload.ashx",
        data: fileData, type: "POST",
        processData: false,
        contentType: false,
        cache: false,
        dataType: "json",
        success: function (mydata) {

             $(document.body).append(mydata);
        },
        error: function(jqXHR, textStatus, errorThrown){
           alert("Status:"+textStatus+"\nError:"+errorThrown);
        }
    });