How does createSlice work and what does it return?

createSlice turns a slice name, an initial state and an object of reducer functions into a slice reducer plus an action creator for each reducer. It returns them in one object, which removes the hand-written action types and switch statements of classic Redux.

How it works:

• Each key in reducers becomes an action type prefixed with the slice name, such as 'counter/increment'.
• Each case reducer is wrapped in Immer, so it can "mutate" state and still produce an immutable update.
• extraReducers adds handlers for actions defined elsewhere, like thunk results, without generating new action creators.

The returned object holds:

• reducer, the slice reducer to pass to configureStore or combineReducers
• actions, an action creator for each case reducer, usually exported by destructuring
• name, the slice name
• getInitialState, a function that returns the initial state
• caseReducers, the original reducer functions
• selectors, for any selectors you define

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {
    increment: (state) => {
      state.value += 1; // Immer makes this safe
    },
    incrementByAmount: (state, action) => {
      state.value += action.payload;
    }
  }
});
// Returns: { reducer, actions: { increment, incrementByAmount } }