Partes do Redux Toolkit e createSlice

npm install @reduxjs/toolkit react-redux

import { createSlice } from '@reduxjs/toolkit';

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {
    increment: state => { state.value += 1; },
    decrement: state => { state.value -= 1; },
    incrementBy: (state, action) => { state.value += action.payload; },
  },
});

export const { increment, decrement, incrementBy } = counterSlice.actions;
export default counterSlice.reducer;

import { configureStore } from '@reduxjs/toolkit';
import counterReducer from './counterSlice';

export const store = configureStore({
  reducer: {
    counter: counterReducer,
  },
});

import { Provider } from 'react-redux';
import { store } from './store';

createRoot(document.getElementById('root')).render(
  <Provider store={store}><App /></Provider>
);

import { useSelector, useDispatch } from 'react-redux';

function Counter() {
  const value = useSelector(state => state.counter.value);
  const dispatch = useDispatch();
  return (
    <>
      <p>{value}</p>
      <button onClick={() => dispatch(increment())}>+</button>
      <button onClick={() => dispatch(incrementBy(5))}>+5</button>
    </>
  );
}

export type RootState = ReturnType<typeof store.getState>;
export type AppDispatch = typeof store.dispatch;
// Then use typed hooks:
const value = useSelector((s: RootState) => s.counter.value);

extraReducers: (builder) => {
  builder.addCase(fetchUser.fulfilled, (state, action) => {
    state.user = action.payload;
  });
}