userreducer-types.ts

type Dispatch<A> = (value: A) => void;
type Reducer<S, A> = (prevState: S, action: A) => S;
type ReducerState<R extends Reducer<any, any>> = R extends Reducer<infer S, any> ? S : never;
type ReducerAction<R extends Reducer<any, any>> = R extends Reducer<any, infer A> ? A : never;

function useReducer<R extends Reducer<any, any>, I>(
  reducer: R,
  initializerArg: I & ReducerState<R>,
  initializer: (arg: I & ReducerState<R>) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];

function useReducer<R extends Reducer<any, any>, I>(
  reducer: R,
  initializerArg: I,
  initializer: (arg: I) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];

function useReducer<R extends Reducer<any, any>>(
  reducer: R,
  initialState: ReducerState<R>,
  initializer?: undefined
): [ReducerState<R>, Dispatch<ReducerAction<R>>];