JavaScript function that could be used to add a product to the shopping cart

function addToCart(productId) {
  // Get the current contents of the shopping cart.
  const cartItems = JSON.parse(localStorage.getItem('cartItems')) || [];

  // Add the new product to the shopping cart.
  cartItems.push({
    id: productId,
    quantity: 1,
  });

  // Save the updated shopping cart to local storage.
  localStorage.setItem('cartItems', JSON.stringify(cartItems));

  // Show a success message to the user.
  alert('Product added to cart!');
}