Reset all reducers back to their initial states when user logout

rootReducer.js

import {combineReducers} from 'redux';
import { LOGOUT } from '../common/constants';
import { UnauthorizedErrorReducer } from '../common/commonReducers';
import FirstReducer from './FirstReducer';
import SecondReducer from './SecondReducer';
import ThirdReducer from './ThirdReducer';

/* In order to reset all reducers back to their initial states when user logout,
 * rewrite rootReducer to assign 'undefined' to state when logout
 *
 * If state passed to reducer is 'undefined', then the next state reducer returns
 * will be its initial state instead; since we have assigned it as the default value
 * of reducer's state parameter
 *   ex: const Reducer = (state = InitialState, action) => { ... }
 *
 * See: https://goo.gl/GSJ98M and combineReducers() source codes for details
 */
const appReducer = combineReducers({
  unauthorized: UnauthorizedErrorReducer,
  first: FirstReducer,
  second: SecondReducer,
  third: ThirdReducer,
});

export default rootReducer = (state, action) => {
  if (action.type === LOGOUT) {
    state = undefined;
  }

  return appReducer(state, action);
};

Haven't written Redux for a while, but I guess the key point is to pass 'undefined' to the reducer which you want to reset the state.
Therefore, you can either call the reducer directly with 'undefined' state or via combineReducers().

What if u want to reset state of just one reducer??

I know this question was posted over a year ago.. I found that this works

case types.LOG_OUT_USER: return { ...INITIAL_STATE };

just define a case called reset_state in the reducer where the action will be to make state = initialstate and dispatch it when you want