Trying to send data from client side but req.session doesn't work properly

I'm trying to do post request, and it is very successful when I do it with postman, but I'm trying to send it from my client-side. I want to post the cart, but as a result I'm constantly posting item with quantity of 1 no matter of fact how many times do I post that request. What is the best solution to fix this and post the request on the normal way?

• I'm using session, maybe that would cause the problem sending request from frontend?

This is my post request:

app.post("/add-to-cart/:id", async (req, res) => {
  try {
      const id = req.params.id;
      
      const { data }  = await axios.get("http://localhost:4200/products");
      const singleProduct = await data.find((product) => product._id === id);

    let cart;
    if (!req.session.cart) {
      req.session.cart = cart = new Cart({});
    } else {
      
      cart = new Cart(req.session.cart);
    }
    req.session.cart = cart;
    cart.addProduct(singleProduct);
    console.log(req.session.cart)
    res.send(cart);
    
  } catch (error) {
    console.log(error);
  }
});

This is the Cart code:

module.exports = function Cart(oldCart) {
  this.productItems = oldCart.productItems || {};
  this.totalQty = oldCart.totalQty || 0;
  this.totalPrice = oldCart.totalPrice || 0.00;

  this.addProduct = function(item) {
    let storedItem = this.productItems;
    if (!storedItem.hasOwnProperty("item")) {
      storedItem = this.productItems = {item: item, qty: 1, price: item.price};
      this.totalQty = 1;
      this.totalPrice = item.price;
    } else {
      storedItem = {item: item, qty: storedItem.qty, price: storedItem.price};
      console.log("STORED ITEM: ", storedItem);
      this.productItems = storedItem;
      storedItem.qty++;
      storedItem.price = storedItem.item.price * storedItem.qty;
      this.totalQty++;
      this.totalPrice += storedItem.item.price;
    }
  }
}

This is the function which I put on my button with @click.prevent="addToCart()"

addToCart(){
      let productId = this.oneProduct.id;
      Cart.postCart(productId);
    }

I'm sending this from axios service on frontend:

postCart(productId){
        return cartService.post(`/add-to-cart/${productId}`);
    }

There are no warnings or errors when I try to interact with this on client side. This is the output which I get in my backend (constantly repeating the information of a single product nevertheless this works normally on the client side):

from Trying to send data from client side but req.session doesn't work properly