A simple use case in hexagonal architecture.

processingCartUseCase.js

const MAX_TOTAL_ITEMS_IN_CART = 10;

const ProcessingCustomerCart = ports => {
  let cartItems = [];
  
  return {
    addItemToCart(item) {
      const cartItemsWithAddedItem = cartItems.concat([item]);
      if (MAX_TOTAL_ITEMS_IN_CART < cartItemsWithAddedItem.length) {
        ports.cartErrorHappened("Too many items in cart. Please proceed to checkout.");
      } else {
        cartItems = cartItemsWithAddedItem;
        ports.cartItemAdded(cartItemsWithAddedItem, item);
      }
    },
    cartItemsConfirmed() {
      ports.cartItemsConfirmed(cartItems);
    }
  };
};