const handleItemDelete = (index) => {
    console.log(index) //<==== why is index is a class?
    dispatch({
      type: "DELETE_ITEM",
      itemId: index,
    });
  };

<ul>
        {items.map((item, index) => (
          <li key={index.toString()}>
            {item}{" "}
            <button
              onClick={handleItemDelete}
            >
              X
            </button>
          </li>
        ))}
      </ul>