Better types, with inference

store.ts

interface Action<T = any> { type: T }
interface AnyAction extends Action { [extraProps: string]: any }

type Reducer<S = any, A extends Action = AnyAction> = (
  state: S | undefined,
  action: A
) => S

interface Store<
  S = any,
  A extends Action = AnyAction,
  StateExt = BaseType<S>,
  Ext = {}
> { {
  getState(): S & StateExt
  dispatch(action: A): A
  replaceReducer<NewState, NewActions extends Action>(
    nextReducer: Reducer<NewState, NewActions>
  ): Store<NewState & StateExt, NewActions, StateExt, Ext> & Ext
}

export type StoreEnhancer<StoreExt = {}, StateExt = {}> = (
  next: StoreEnhancerStoreCreator<StoreExt, StateExt>
) => StoreEnhancerStoreCreator<StoreExt, StateExt>

export type StoreEnhancerStoreCreator<StoreExt = {}, StateExt = {}> = <
  S = any,
  A extends Action = AnyAction
>(
  reducer: Reducer<S, A>,
  preloadedState?: S
) => Store<S & StateExt, A, StateExt, Ext> & StoreExt