单击添加到购物车时，购物车无法正常工作

Question

我正在尝试在项目中构建购物车系统。我为此使用django和javascript。但是当我单击“添加到购物车”时，产品并未添加到购物车

这是我的views.py

def updateCart(request):
    data = json.loads(request.body)
    productId = data['productId']
    action = data['action']
    print('productId:',productId)
    print('Action:',action)

    customer = request.user.customer_name
    product = ProductModel.objects.get(product_id=productId)
    order, created = OrderModel.objects.get_or_create(customer=customer, complete=False)
    orderItem, created = orderItem.objects.get_or_create(order=order, product=product)

    if action == 'add':
        orderItem.quantity = (orderItem.quantity + 1)
    elif action == 'remove':
        orderItem.quantity = (orderItem.quantity - 1)

    orderItem.save()

    if oderItem.quantity <= 0:
        orderItem.delete()


    return JsonResponse('Item has been added to cart', safe=False)

这是我的cart.js

var updateBtn = document.getElementsByClassName('update-cart')

for(var i = 0; i < updateBtn.length; i++){
    updateBtn[i].addEventListener('click', function(){
        var productId = this.dataset.product
        var action = this.dataset.action
        console.log('productId:',productId, 'action:',action)

        console.log('USER:',user)
        if(user === 'AnonymouseUser'){
            console.log('Please Login to Order!')
        }
        else {
            updateUserOrder(productId, action)
        }
    })
}

function updateUserOrder(productId, action){
    console.log('Product added to cart!')

    var url = '/update_item/'

    fetch(url, {
        method: 'POST',
        headers: {
            'content-Type': 'application/json',
            'X-CSRFToken': csrftoken
        },

        body: JSON.stringify({'productId': productId, 'action':action})
    })

    .then((response) => {
        return response.json()
    })

    .then((data) => {
        console.log('data:',data)
        // location.reload()
    })

}

我已经添加了csrf令牌，但仍然遇到此错误。我修改了很多次代码，但找不到主要问题。我不知道该怎么办，我被困在这里：（

这是我遇到的错误：

POST http://127.0.0.1:8000/update_item/ 500 (Internal Server Error)
updateUserOrder @ cart.js:24
(anonymous) @ cart.js:14
(index):1 Uncaught (in promise) SyntaxError: Unexpected token < in JSON at position 0