how to get specific value from server response for uploaded file ajax

i have tried to send uploaded file to server by using ajax like this

    formData.append('foldername',fname);
    formData.append('file', file)
    $.ajax({

            url: 'imageupload',
            data: formData,
            processData: false,
            contentType: false,
            type: 'POST',
            success:function(response)
            {


                            alert(response);
                            alert(response.imagename);


            }
    });

it is send data to server successfully but i have sent response from server like this

  res.writeHead(200,{'Content-Type':'text/html', 'Access-Control-Allow-Orgin':'*'});
  res.write(JSON.stringify({"imagename":"1.jpeg","imageid":"xxxxxxxxxxxxxxxxxxx"}));
  res.end();

i have written two alert in success function . in first alert i got like this

{"imagename":"1.jpeg","imageid":"xxxxxxxxxxxx"}

i have written second alert for to get imagename but i got undefine

so i could not get specific key value. how to resolve this?

change 

res.writeHead(200,{'Content-Type':'text/html', 'Access-Control-Allow-Orgin':'*'});

to 

res.writeHead(200,{'Content-Type':'application/json', 'Access-Control-Allow-Orgin':'*'});

jquery don't convert a JSON string to Javascript Object if the content type is 'Content-Type':'text/html'. So, 'Content-Type':'application/json' or 'Content-Type':'text/json' will do the magic. You will get the response as Javascript object in the client. So, you can do 

alert(response.imagename);//alerts image name