Dynamic progress bar based on script progress?

I have a Google App Engine application using the webapp2 framework that interacts with a MySQL database. Application users can upload data. I want to show a progress bar while loading, as it can take up to several minutes.
Based on what I've seen in other threads (mainly: this section and this one , m working on a JSON / Javascript solution that is new to me.

The progress bar itself works if I pass in a random number. However, I cannot figure out how to "load" the changing values ​​from a Python script.

Here's the HTML / CSS / Javascript:

HTML:
<div id="myProgress">
    <div id="myBar"</div>
</div>  

CSS:
#myProgress {width: 300px;
height: 30px;
background-color: #ddd;
}
#myBar {width: 1%;
height: 30px;
background-color: #4CAF50;
}   

Javascript:     
<script type="text/javascript">
function move() {
    var elem = document.getElementById("myBar");   
    var width = 1;
    var id = setInterval(frame, 1000);
    function frame() {
        if (width >= 100) {
            clearInterval(id);
        } 
        else {
            //var randomnumber = Math.floor(Math.random() * 100); --> works
            var randomnumber = function update_values() {
                $SCRIPT_ROOT = {{ script_root }};
                $.getJSON($SCRIPT_ROOT+"/uploading",
                    function(data) {
                        $("#width").text(data.width+" %")
                    });
            } ; --> this is wrong I assume
            var width = randomnumber; 
            elem.style.width = width + '%'; 
        }
    }
}
window.onload=move();   
</script>

      

        
        
        
      

    

The progress comes from a for loop in Python, which is embedded in the script that loads the page. After the script finishes in one action, I want the counter result to be passed to the progress bar as its width. With static variables, I use the usual Jinja way.

class UploadingpageHandler(webapp2.RequestHandler):
def get(self):
    activities_list = [1,2,3,4,5,6,7,8,9,10]
    counter = 0
    script_root = 'localhost:10080'

    for activity in activities_list:
        counter = counter + 10
        upload.do_stuff_in_some_function_with_MySQL()   
        obj = {
        'width': counter
        }
        self.response.headers['Content-Type'] = 'application/json' --> this
        self.response.out.write(json.dumps(obj)) --> and this is wrong I assume

    template_vars = {
    'script_root': script_root
    }
    template = jinja_environment.get_template('uploading.html')
    self.response.out.write(template.render(template_vars))

      

        
        
        
      

    

How do I change the scripts to make it work? Or is there a better way to solve this?