How to render a loading page in a website while waiting for response

I am currently building a web application using NodeJS and express. One feature of this application is to have a user upload images (which is then stored in a cloud server).
However, i find that this sometimes takes a lot of time and the server is waiting for a response. I tried adding a loading screen but that is only triggered when the website is loading, and not while it's waiting for a response (as in the above case). Here is the code for the loading screen

<div class="loader">
   <img src="/loading.gif" alt="Loading..." />
</div>

<script>
        window.addEventListener("load", function () {
        const loader = document.querySelector(".loader");
        loader.className += " hidden"; // class "loader hidden"
    });
</script>

There's some CSS involved which I'm not posting here, but this loading screen works fine.
Here is the part that handles the request.

router.post('/campgrounds',middleware.isLoggedIn, upload ,(req,res)=>{
    //BELOW PROCESS TAKES LOT OF TIME
    //ADD A LOADING SCREEN HERE TILL IT REACHES "SUCCESS"
    cloudinary.uploader.upload(req.file.path, function(error, result) {
    if(error) {
        req.flash('error', err.message)
        return res.redirect('back')
    }
    // add cloudinary url for the image to the campground object under image property
    req.body.campground.image = result.secure_url;
    req.body.campground.imageId = result.public_id;
    req.body.campground.author = {
        id: req.user._id,
        username: req.user.username
    }
    Campground.create(req.body.campground, function(err, campground) {
        if (err) {
        req.flash('error', err.message);
        return res.redirect('back');
        }
        console.log("SUCCESS")
        req.flash("success","Successfully posted")
        res.redirect('/campgrounds/' + campground.id);
    });
    });
})

As mentioned, i want to render a loading screen while it uploads. How do i do this while the client is waiting for a response?
Any help is much appreciated