I have a JS function in my template which works with fetch commands to relay to a Django view. The strange part is that it as expected most of the time but sometimes it just doesn't and I cannot figure out why.

JS in template:

<script>
    if (document.querySelectorAll(".nav-link")[1].innerHTML != "Log In") {
        var like = document.querySelector(".likes");
        var quantity = document.querySelector(".quantity");
        like.addEventListener("click", () => likes())
        function likes () {
            fetch(`/likes/${like.value}`, {
                method: "PUT"});
            if (like.innerHTML.slice(0, 2) != "Un") {
                like.innerHTML = "Unlike &#128148";
            }
            else {
                like.innerHTML = "Like &#10084";
            }
            fetch(`/likes/${like.value}`)
            .then(response => response.json())
            .then(post => {
                console.log(post["liked"]);
                quantity.innerHTML = post["liked"].length;
            });
        }
    }
</script>

my view:

@csrf_exempt
@login_required
def likes(request, id):
    user = User.objects.get(username=request.user)
    try:
        post = Post.objects.get(pk=id)
    except Post.DoesNotExist:
        return JsonResponse({"error": "No such post"}, status=404)
    
    if request.method == "GET":
        return JsonResponse(post.serialize())

    if request.method == "PUT":
        #data = json.loads(request)
        if user not in post.liked.all() and post.poster != user:
            print("adding")
            post.liked.add(user)
            post.save()
        else:
            print("removing")
            post.liked.remove(user)
            post.save()
        return HttpResponse(status=204)

It should be alternating between an empty list and a list which contains a two.

However you can see that, although it usually works right, it has a spot where there are several "[2]" in a row and sometimes it has several "[]" in a row too.

Actually this only started after adding the second fetch but I can't understand why; before then it alternated without problems.

CodePudding user response：

As @Jared Smith said in comments there is no guarantee order of fetch as it is asynchronous, to control the order use async and await to wait for the result of response something like this:

    like.addEventListener("click", async () => await likes())
    async function likes () {
        await fetch(`/likes/${like.value}`, {
            method: "PUT"});
        if (like.innerHTML.slice(0, 2) != "Un") {
            like.innerHTML = "Unlike &#128148";
        }
        else {
            like.innerHTML = "Like &#10084";
        }
        await fetch(`/likes/${like.value}`);
        const post = await response.json();
        quantity.innerHTML = post["liked"].length;
    }