64Implement a small Redux-style store using subscriptions and reducers

Implement the core of Redux and a React binding:

• createStore(reducer, preloadedState) returns { getState, dispatch, subscribe }.
  • On creation, dispatch an internal { type: "@@INIT" } action so reducers can supply their default state.
  • dispatch(action) runs the reducer, stores the new state, calls every subscriber, and returns the action.
  • subscribe(listener) returns an unsubscribe function.
• combineReducers({ key: reducer, … }) builds one reducer from several slice reducers.
• useSelector(store, selector) returns selector(store.getState()) and re-renders the component only when that selected value changes.

const store = createStore(combineReducers({ todos, filter }));
function Count() {
  const count = useSelector(store, (state) => state.todos.length);
  return <span>{count}</span>; // does not re-render when only the filter changes
}
store.dispatch({ type: "todos/add", text: "Ship it" });

Requirements

• Actions must be plain objects with a string type; otherwise throw a TypeError.
• Reducers may not dispatch: calling dispatch while a reducer runs throws an Error.
• A listener that unsubscribes (or subscribes) during a dispatch does not change who is notified for that dispatch.
• combineReducers returns the same state object when no slice changed.