Hi Team

I am unable to calculate total to my shopping cart and discount, when a user add item on the basket(this shows both incrementing and decrementing). The problem is on getting the discount and total of those item being on the basket. Currently i can only see Nan as total need some help. 

// php code
 $quantity) {
    $_SESSION['cart'][$product_id] = $quantity;
  }
  $total = $_POST['total'];
  echo json_encode(array('success' => true, 'total' => $total));
  exit;
}

?>




// jquery code
// Get the cart from local storage
function getCart() {
  var cart = localStorage.getItem('cart');
  if (cart) {
    return JSON.parse(cart);
  } else {
    return {};
  }
}

// Save the cart to local storage
function saveCart(cart) {
  localStorage.setItem('cart', JSON.stringify(cart));
}

// Update the cart count in the navbar
function updateCartCount() {
  var cartCount = Object.values(getCart()).reduce((a, b) => a + b, 0);
  $('#cart-count').text(cartCount);
}

// Add a product to the cart
function addToCart(productId, quantity) {
  var cart = getCart();
  if (cart.hasOwnProperty(productId)) {
    var newQuantity = parseInt(quantity);
    if (newQuantity <= 0) {
      delete cart[productId];
    } else {
      cart[productId] = newQuantity;
    }
  } else {
    cart[productId] = parseInt(quantity);
  }
  saveCart(cart);
  updateCartCount();
  calculate_total();

  // Display message
  var productName = $('#product-name-' + productId).text();
  $('#cart-message').text(quantity + 'x ' + productName + ' added to cart').show();
  
  setTimeout(function() {
    $('#cart-message').fadeOut('slow');
  }, 3000);
}

// Calculate and display the total amount in the shopping cart
function calculate_total() {
  var cart = getCart();
  var total = 0;
  for (var productId in cart) {
    var product = getProductId(productId);
    var price = parseFloat(product.price);
    var quantity = parseInt(cart[productId]);
    var subtotal = price * quantity * (1 - parseFloat(product.discount));
    total += subtotal;
  }
  $('#total-amount').text('$' + total.toFixed(0)); // update the text of the total td element
  $('#total').val(total.toFixed(0));
  return total.toFixed(0);
}


// get the product details.
function getProductId(productId) {
  return $.ajax({
    url: 'get_product_details.php',
    data: { productId: productId },
    dataType: 'json'
  });
}

// display data from the cart.

function display_cart() {
  var cart = getCart();
  var $cart_table = $('#cart-table');

  // Remove existing rows from the table
  $cart_table.find('tbody tr').remove();

  // Add new rows to the table for each product in the cart
  for (var productId in cart) {
    $.ajax({
      url: 'get-product-details.php',
      data: { productId: productId },
      success: function(product) {
        var price = parseFloat(product.unit_price);
        var quantity = parseInt(cart[product.productId]);
        var subtotal = price * quantity;
        var discount = parseFloat(product.discount);
        var total = subtotal - (subtotal * discount);
        var $row = $('').appendTo($cart_table.find('tbody'));
        $('').append($('').attr('src', 'img/detailsquare.jpg').attr('alt', product.product_name)).appendTo($row);
        $('').append($('').attr('', '').text(product.product_name)).appendTo($row);
        $('').text(quantity).appendTo($row);
        $('').text('$' + price.toFixed(2)).appendTo($row);
        $('').text((discount * 100) + '%').appendTo($row);
        $('').text('$' + total.toFixed(2)).appendTo($row);
      },
      error: function(xhr, status, error) {
        console.error('Error getting product details:', error);
      }
    });
  }

  // Update the total amount in the cart
  calculate_total();
}


$(document).ready(function() {
  $('.btn-plus').click(function() {
    var productId = $(this).closest('.input-group').find('.product-quantity').data('product-id');
    var quantityInput = $(this).closest('.input-group').find('.product-quantity');
    var currentQuantity = parseInt(quantityInput.val());
    var newQuantity = currentQuantity + 1;
    quantityInput.val(newQuantity);
    addToCart(productId, newQuantity);
  });

  $('.btn-minus').click(function() {
    var productId = $(this).closest('.input-group').find('.product-quantity').data('product-id');
    var quantityInput = $(this).closest('.input-group').find('.product-quantity');
    var currentQuantity = parseInt(quantityInput.val());
    if (currentQuantity > 1) {
      var newQuantity = currentQuantity - 1;
      quantityInput.val(newQuantity);
      addToCart(productId, newQuantity);
    }
  });

  // Initialize the cart count in the navbar
  updateCartCount();
});



// html code

              

                

                  
Shopping cart
                  
You currently have 
                  

                  

                    

                      

                        

                          
Product
                          
Quantity
                          
Unit price
                          
Discount
                          
Total

                        
                      
                      

                        

                          

                          
White Blouse Armani
                          

                

                
                -
                
                
              
              +
            
          
                          
                          
R
                          

                          
R
                          

                        
                        

  

  
White Blouse Armani
  

    

      
        -
      
      
      
        +
      
    
  
  
R
  
%13
  
R
  



                      
                      

                        

                          
Total
                          

                          
                        
                      
                    
                  
                  

                  
                  

                    
 Continue shopping
                    

                       Update cart
                      Proceed to checkout