Requirement:

There is form in my html page where we can upload files and post them. After posting the files I wish to save it in the backend and then proceed to the next page of my application in a single click.

my javascript code for submit/posting this form.

submitButton.addEventListener("click", function(event) {
                    event.preventDefault();
                    var formData = new FormData(form);  
                    var redirect_url = '{{ url_for("UploadLoads") }}?link=' + link;                  
                    fetch(redirect_url , {
                        method: 'POST',
                        body: formData               
                    })
                    .then(response => {
                        const contentType = response.headers.get("content-type");
                        if (contentType && contentType.includes("application/json")) {
                            return response.json().then(data => {
                                throw new Error(data.message); }  
                        else {
                            throw new Error("Unknown error occurred");
                        }
                    })
                    .then(data => {
                                window.location.href = link;                                 
                        })
                    .catch(error => {
                        console.error('Error:', error);
                    });
                    window.location.href = link;  
                });

flask code:

@app.route('/loadsupload', methods=['POST'])

def UploadLoads():
    
    if request.method=='POST':
        link = request.args.get('link') 
        if "date_time" not in session or session["date_time"] == None:

            session["date_time"]        = datetime.now().strftime("%Y_%m_%d_%H_%M_%S")
            
            if session["date_time"] not in os.listdir(get_session_data("user_dir")):
                os.mkdir(get_session_data("session_dir"))
                description=[]
                for i in range(1,len(request.files)+1):
                    file = request.files.get(f'file{i}')
                    if file:
                        filename = os.path.join(get_session_data("session_dir"), f'file{i}.xlsx')
                        file.save(filename)
                    text = request.form.get(f'description{i}', '')
                    description.append(text)
                session['Optitower load description']=description
                session['Optitower load family']=request.form['family']
                
    return jsonify({'redirect': link}), 200
    @app.route('/CoverForm/<path:variable>', methods=['GET', 'POST'])
    @UserPathChecks(check_name = "User Session")
    # @GetTime(name="Cover Form Without Userpath", level=" Coverform ")
    def CoverForm(variable):           
       tenders = None
       try:
         
        tenders = tender_data()

       except:
        print("not able to fetch tenders")
...............................................

Logic Explaination

I am using this UploadLoads() function to get the uploaded files and save it the backend. The link variable has url path which I would like to go to as soon as I have exited my UploadLoads() function meaning as soon I as my files are saved, I want my application to go the next page specified by ‘link’ variable.
example of link
link=”CoverForm/” +brd + “/” + url

Problem that I am facing

Sometimes if a click on submit button this uploadloads get activated and my files are saved the way I want them to but the page doesnt move ahead. The second time I hit the submit button the page moves forward skipping the file saving function. So I end up getting two folders in the backend first one which has the files and second which some other data. I want both saving the files and next page to happen in a single click only