Is it possible to modify a store from another store? - reactjs

Question

I am using redux with multiple reducers combined into rootReducer. How is it possible to modify the state of one reducer from another reducer? Ex:
// systemReducer.js
const INITIAL_STATE = { isLoggedIn: true }
function systemReducer(state = INITIAL_STATE, action) {
switch(action.type) { ... }
}
// messagesReducer.js
const INITIAL_STATE = { messages: [] }
function messagesReducer(state = INITIAL_STATE, action) {
switch(action.type) { ... }
}
And then say I have action makers for messagesReducer as under:
// messageActions.js
export const messagesFetchAction = (data) => {
return {
type: MESSAGES_FETCH,
data: data
}
}
Now, how can I modify systemReducer's isLoggedIn to false from messagesFetchAction()? So it looks like as under for example:
// messageActions.js
export const messagesFetchAction = (data) => {
systemState.setState({isLoggedIn: false}); // <=====
return {
type: MESSAGES_FETCH,
data: data
}
}

Answer 1

If you have two reducers does not mean that you have several stores. You still have single store, but combined from two reducers. In general, your store may look like:
{
systemReducer: {
isLoggedIn: true
},
messagesReducer: {
messages: []
}
}
You couldn't dispatch action from reducer. This is prohibited by redux. But you may dispatch several actions from action creator. For example:
export const messagesFetchAction = (data) => (dispatch) => {
dispatch({type: LOGGED_IN, isLoggedIn: false});
dispatch({
type: MESSAGES_FETCH,
data: data
});
}
The action creator above is for Redux Thunk. To be able to use it, apply middleware when creating store like this
const rootReducer = combineReducers({
systemReducer,
messagesReducer
});
const store = createStore(rootReducer, applyMiddleware(
thunkMiddleware
));