Used in https://medium.com/@timdeschryver/ngrx-immer-7fe4a0d43508

cart.immer-reducer.ts

export const reducer = produce<State, CartActions>((draft, action) => {
  switch (action.type) {
    case CartActionTypes.AddToCart:
      draft.cartItems[action.payload.sku] = (draft.cartItems[action.payload.sku] || 0) + 1;
      return;

    case CartActionTypes.RemoveFromCart:
      const newAmount = draft.cartItems[action.payload.sku] - 1;
      if (newAmount > 0) {
        draft.cartItems[action.payload.sku] = newAmount;
        return;
      }
      delete draft.cartItems[action.payload.sku];
      // or
      // draft.cartItems[action.payload.sku] = 
      //   Math.max((draft.cartItems[action.payload.sku] || 0) - 1, 0);
      return;

    case CartActionTypes.EmptyCart:
      return initialState;
  }
}, initialState);