I'm sure there is a better way of doing this...

  function increaseItemQuantity(cartItem: Item) {
    setCart((items) => {
      const cartItemExists = items.find((item) => item.id === cartItem.id);

      if (!cartItemExists) {
        const newItem: CartItem = {
          id: cartItem.id,
          quantity: cartItem.quantity,
          variants: [cartItem.variant],
        };

        return [...items, newItem];
      }

      return items.map((item) => {
        if (item.id === cartItem.id) {
          const existingVariant = item.variants.find(
            (variant) => variant.name === cartItem.variant.name
          );

          if (!existingVariant) {
            return {
              ...item,
              quantity: item.quantity + cartItem.quantity,
              variants: [
                ...item.variants,
                { name: cartItem.variant.name, quantity: 1 },
              ],
            };
          }

          return {
            ...item,
            quantity: item.quantity + cartItem.quantity,
            variants: item.variants.map((variant) =>
              variant.name === cartItem.variant.name
                ? {
                    name: variant.name,
                    quantity: variant.quantity + cartItem.quantity,
                  }
                : variant
            ),
          };
        }

        return item;
      });
    });
  }

Comments

[Onorio Catenacci]

Welcome to Dev.to!

Not trying to discourage you or anything like that but I think you may get better assistance if you share your code on a website devoted to peer review. Perhaps this one: codereview.stackexchange.com Or if you have some specific idea about what you think could be improved, perhaps you could tell us and we can offer suggestions to you.

[joshuamwo]

Thanks.