'<', "<!DOCTYPE "... is not valid JSON Promise.then (async) error in Django [closed]

I m currently learning Django and i m trying to build an ecommerce website. I m trying to figure out why am i getting this error in my console.I m trying to do a fetch call and trying to send data to the url by name of update_item and i m expecting the view function to return the json response in my console but i m constantly getting this error. Can someone pls help.

**This is my js file. **

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

for (i = 0; i < updateBtns.length; i++) {
    updateBtns[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 == 'AnonymousUser'){
            console.log('User is not authenticated')

        }else{
            updateUserOrder(productId, action)
        }
    })
}

function updateUserOrder(productId, action){
    console.log('User is authenticated, sending data...')

        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)
        });
}

This is my views.py file.

from django.shortcuts import render
from django.http import JsonResponse
from .models import *

# Create your views here.
def store(request):
    products = Product.objects.all()
    context = {'products':products}
    return render(request,'store/store.html',context)

def cart(request):
    if request.user.is_authenticated:
        customer = request.user.customer
        order, created = Order.objects.get_or_create(customer=customer, complete=False)
        items = order.orderitem_set.all()
    else:
        # Create empty cart for now for non-logged in user
        items = []
        order = {'get_cart_total':0, 'get_cart_items':0 }

    context = {'items': items, 'order':order}
    return render(request, 'store/cart.html', context)

def checkout(request):
    if request.user.is_authenticated:
        customer = request.user.customer
        order, created = Order.objects.get_or_create(customer=customer, complete=False)
        items = order.orderitem_set.all()
    else:
        # Create empty cart for now for non-logged in user
        items = []
        order = {'get_cart_total':0, 'get_cart_items':0 }

    context = {'items': items, 'order':order}


    return render(request,'store/checkout.html',context)

def updateItem(request):
    return JsonResponse('Item was added', safe=False)

This is my urls.py file

from django.urls import path
from . import views

urlpatterns = [
    path('', views.store, name = "store"),
    path('cart/', views.cart, name = "cart"),
    path('checkout/', views.checkout, name = "checkout"),
    path('update_item/', views.updateItem, name = "update_item")
]

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