ActionDispatcher.ts

interface Dispatch<A extends { type: string }> {
  (action: A): void;
}

interface StateGetter<S extends { [key: string]: any }> {
  (): S;
}

interface ActionReducer<A extends { type: string }, S extends { [key: string]: any }> {
  dispatch: Dispatch<A>;
  getState: StateGetter<S>;
}

function ActionReducer<A extends { type: string }, S extends { [key: string]: any }>(
  reducer: Reducer<S, A>,
  initialState: S,
): ActionReducer<A, S> {
  let state = initialState;

  return {
    dispatch(action) {
      state = reducer(state, action);
    },
    getState: () => ({ ...state }),
  };
}