I was able to get it working using sessions:

<a id="page" class="page-link">{{ num }}</a>

let page=document.querySelector("#page").innerHTML
            fetch(`/edit/${content[i].id}`,
            {
                method: 'POST',
                headers: {'X-CSRFToken': csrftoken},
                mode: 'same-origin',
                body: JSON.stringify({
                post: textarea[i].value,
                page: page             
                })}).then(() => {
                    editdiv[i].style.display = 'none';
                    post[i].style.display = 'block';
                })})

def edit(request, post_id):
    data = json.loads(request.body)
    content = data.get("post", "")
    post=Post.objects.get(id=post_id)
    page = data.get("page", "")
    request.session['page'] = page
    if request.method == "POST":
        if post.user == request.user:
            post.post=content
            post.save()
            return HttpResponse(page)

def index(request):
    posts = Post.objects.all().order_by('-date')
    paginator = Paginator(posts,10)
    page_number = request.GET.get('page')
    if request.session.has_key('page'):
        page_number = request.session['page']
        del request.session['page']

reverse(…) ^[Django-doc] looks for a view with the given name, it does not take a URL as input. You can use this directly in the HttpResponseRedirect ^[Django-doc], so:

def edit(request, post_id):
    data = json.loads(request.body)
    content = data.get("post", "")
    post=Post.objects.get(id=post_id)
    page = data.get("page", "")
    if request.method == 'POST':
        if post.user == request.user:
            post.post=content
            post.save()
            return HttpResponseRedirect(page)

You should also return HttpResponses for the GET requests, and in case the post.user is not the same as the request.user.

Note: It is often better to use get_object_or_404(…) ^[Django-doc], then to use .get(…) ^[Django-doc] directly. In case the object does not exists, for example because the user altered the URL themselves, the get_object_or_404(…) will result in returning a HTTP 404 Not Found response, whereas using .get(…) will result in a HTTP 500 Server Error.

Note: You can limit views to a view to authenticated users with the @login_required decorator ^[Django-doc].