Redux Store

Requirements

• Export createStore(reducer, preloadedState?) as named and default. Return { getState, dispatch, subscribe }. Initialize by calling reducer with the supplied state, or undefined, and { type: "@@finefrontend/INIT" }.
• Reducers must return a defined state and tolerate unknown actions, including initialization. dispatch(action) accepts a non-null, non-array object with a string type, updates state synchronously, notifies listeners, and returns the exact action.
• subscribe(listener) registers a zero-argument listener and returns an idempotent unsubscribe function. Duplicate callbacks are independent registrations. Do not notify when subscribing; notify after every successful dispatch, even if state identity is unchanged.
• Notify a snapshot in registration order. Listeners added during notification wait until the next dispatch; listeners removed during notification still run in the current snapshot. Listener dispatch is allowed and sees the latest state.
• Disallow dispatch, getState, subscribe, and unsubscribe while a reducer is running. Propagate reducer errors, keep the previous state, and allow later valid dispatches. A reducer returning undefined is an error.
• Listener errors propagate and stop current notification, while the already updated state remains committed. Keep instances independent. Middleware, enhancers, replaceReducer, immutability enforcement, and full Redux action validation are outside scope.

Examples

Input: const store = createStore((state = 0, action) => action.type === "increment" ? state + 1 : state);
const unsubscribe = store.subscribe(() => console.log(store.getState()));
store.dispatch({ type: "increment" });
unsubscribe();
Output: State is 1; the listener ran once.

Input: store.getState();
Output: Returns the current state synchronously.