Redux Toolkit steps

create src/state/store.ts with configureStore()

import { configureStore } from "@reduxjs/toolkit"
import rootReducer from "./reducers" // /index.ts

const store = configureStore({
  reducer: {
    //reducer_key: reducer
  }
})

// // Infer & Export types for RootState & AppDispatch
export type RootState = ReturnType<typeof store.getState\>
export type AppDispatch = typeof store.dispatch

Provide store to <App>

... root.render(
  <StrictMode\>
    <Provider store={store}\>
      <App /\>
    </Provider\>
  </StrictMode\>)

Create reducers inside newer "slice" in src/state/reducers
to get "slice" of state - define initState & reducers applies immer so now can mutate state directly!

import { createSlice } from "@reduxjs/toolkit"
import type { PayloadAction } from "@reduxjs/toolkit"
import { RootState } from "../store"

interface CounterState {
  value: number
}

const initialState: CounterState = {
  value: 0,
}

export const counterSlice = createSlice({
  name: "counter",
  // `createSlice` will infer the state type from the `initialState` argument
  initialState,
  reducers: {
    increment: (state) => {
      state.value += 1
    },
    decrement: (state) => {
      state.value -= 1
    },
    // Use the PayloadAction type to declare the contents of `action.payload`
    incrementByAmount: (state, action: PayloadAction<number\>) => {
      state.value += action.payload
    },
  },
})

export const { increment, decrement, incrementByAmount } = counterSlice.actions

// Other code such as selectors can use the imported `RootState` type
export const selectCount = (state: RootState) => state.counter.value

export default counterSlice.reducer

References