How to make data of a cart to increase dynamically?

I'm trying to make cart items to stack dynamically every time on function call: addProduct(). The data which I want to work with was captured successfully but the problem is that on each function call the quantity always remains 1.

This is the logic which I implemented:

module.exports = function Cart(oldCart) {
    this.productItems = oldCart.productItems || {};
    this.totalQty = oldCart.totalQty || oldCart.totalQty==0.00;
    this.totalPrice = oldCart.totalPrice || oldCart.totalPrice==0.00;
    
    this.addProduct = function(item, id) {
        
        //let storedItem = this.productItems = {item: item, qty: 0, price: 0}; 
        /*Using this storedItem qty: 0, price: 0 for testing*/
   
        if (!storedItem){
            storedItem = this.productItems = {item: item, qty: 0, price: 0};
        }
        storedItem = {item: item, qty: storedItem.qty, price: storedItem.price}

        storedItem.qty++;
        storedItem.price = storedItem.item.price * storedItem.qty;
        this.totalQty ++;
        this.totalPrice += storedItem.price;
        
        //this.productItems += storedItem;
        console.log("Product items: ",this.productItems)
        
    }

    this.generateArray = function () {
        let arr = [];
        for (let id in this.productItems) {
            arr.push(this.productItems);
        }
        return arr;
    }};

When I remove line: let storedItem = this.productItems = {item: item, qty: 0, price: 0}; the result remains NaN. So how to solve this problem? Because the items are still at the quantity of 1 no matter how many API calls do I make.

Here is my app.post call:

app.post('/add-to-cart/:id', async (req, res) => {
  try {
    let id = req.params.id;
    let cart = new Cart(req.body.cart ? req.body.cart : {});

    const { data } = await axios.get('http://localhost:4200/products');
    const singleProduct = await data.find((product) => product._id === id)
    
    
    
    cart.addProduct(singleProduct, singleProduct._id)
    req.body.cart = cart;
    res.redirect('/');
    console.log("console log cart",cart);
    console.log(req.body);
  } 
  
  catch (error){
    console.log(error)
  }
  
});

Here is my console.log result (every time the same):

How to solve this issue?