optimistic-cart-reducer.ts

export function reducer(
  state: CartState = initState,
  action: cartActions.All
): CartState {
  switch (action.type) {
    case cartActions.ADD_ITEM: {
      // Concatinating the id to the list
      const newCartItemsIds = [...state.cartItemsIds, action.itemId];
      return {
        cartItemsIds: newCartItemsIds,
      };
    }
    case cartActions.ADD_ITEM_ERROR: {
      const indexOfItemId = state.cartItemsIds.indexOf(action.itemId);
      // Force array to be "immutable" (cloning the one in the state).
      const newCartItemsIds = [...state.cartItemsIds];
      // Remove the element.
      newCartItemsIds.splice(indexOfItemId, 1);
      
      return {
        cartItemsIds: newCartItemsIds,
      };
    }
...
  } 
  return state;
}