Uncaught Error: "reducer" is a required argument, and must be a function or an object of functions that can be passed to

I have done hours of debugging and I am unable to find the solution.

import { combineReducers, applyMiddleware } from 'redux';
import thunk from 'redux-thunk';
import { configureStore } from '@redux.js/toolkit';
import { composeWithDevTools } from 'redux-devtools-extension';

let reducer = combineReducers({
  // contains all reducers
});

// here all our data will lie
const initialState = {};

const middleware = [thunk];

const store = configureStore(
  reducer,
  initialState,
  composeWithDevTools(applyMiddleware(...middleware))
);

export default store;

Error:

I just want a todo list to be displayed on browser, which I have written in my App.js.

Try:

const store=configureStore(
    {
        reducer,
        initialState,
        composeWithDevTools( ... )
    }
)

export default store;

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