Ajax methods to manage cart items in Shopify

shopifyCartAjaxify.js

    // Add item to the cart
    function cartItemAdd(id, qty) {
      fetch('/cart/add.js', {
        method: 'POST',
        credentials: 'same-origin',
        headers: {
          'Content-Type': 'application/json',
          'X-Requested-With': 'XMLHttpRequest'
        },
        body: JSON.stringify({'items': [{'id':id,'quantity':qty}]})
      })
      .then(response => {
        return response.json();
      })
      .catch((error) => {
        console.error('Error:', error);
        alert('cartAdd');
        alert(error);
      });
    }