Add "ReducerState" type to definition file

[septs]

import { combineReducers } from "redux";

import global, { GlobalState } from "./global";
import foo, { FooState } from "./global";
import bar, { BarState } from "./global";

export default combineReducers({
    global,
    foo,
    bar
});

export interface RootState {
    global: GlobalState,
    foo: FooState,
    bar: BarState,
}

import { combineReducers, Reducer } from "redux";

declare type ReducerState<T> = T extends Reducer<infer S> ? S : never;

export const reducer = combineReducers({
    global,
    foo,
    bar,
});

// get global internal state typing
export type GlobalState = ReducerState<typeof global>;
// get root state with combine all state typing
export type RootState = ReducerState<typeof reducer>;

This can greatly reduce the writing of boilerplate code.

RootState can be automatically generated.

---

infer keyword release for TypeScript 2.8

[timdorr]

This was already added via #3411. It hasn't been released yet.