product-reducer.ts

export function reducer(
  state: ProductState = initState,
  action: actions.All
): ProductState {
  switch (action.type) {
    ...
    case actions.FETCH_PRODUCTS_SUCCESS: {
      return {
        // addAll replaces current list of products with new list
        products: productAdapter.addAll(action.payload, state.products),
        isLoading: false,
      };
    }

    case actions.FETCH_PRODUCT_SUCCESS: {
      return {
        ...state,
        // upsertOne adds or updates single product with payload data
        products: productAdapter.upsertOne(action.payload, state.products),
      };
    }
  ...
  }
}