combineReducers.js

// @flow
declare type Reducer<S, A> = (state: S, action: A) => S;
type ExtractState = <S>(r: Reducer<S, *>) => S;
declare function combineReducers<O, A>(reducers: O): Reducer<$ObjMap<O, ExtractState>, A>;

type State = {
  name: string,
  age: number
};

type Action = any;

function reducerName(name: string, action: Action): string {
  return name
}

function reducerAge(age: number, action: Action): number {
  return age
}

// ok
const reducer0: Reducer<State, Action> = combineReducers({
  name: reducerName,
  age: reducerAge
})

// error
const reducer1: Reducer<State, Action> = combineReducers({
  name: reducerName
  // missing age reducer
})

// error
const reducer2: Reducer<State, Action> = combineReducers({
  name: reducerName,
  age: reducerName // wrong reducer
})

// error
const reducer3: Reducer<State, Action> = combineReducers({
  a: 1, // unknown key
  name: reducerName,
  age: reducerAge
})

// error
const reducer4: Reducer<State, Action> = combineReducers({
  name: () => undefined, // bad reducer
  age: reducerAge
})