Creating your Application's Root Reducer

@ngrx allows us to break our application into smaller reducers with a single area of concern. We can combine these reducers by creating an object that mirrors the application's AppState, where each property will point to one of those smaller reducers.

app/store/rootReducer.ts

import {counterReducer} from './counter/counter.reducer';
export const rootReducer = {
  counter: counterReducer
};

原文: https://angular-2-training-book.rangle.io/handout/state-management/ngrx/creating_your_applications_root_reducer.html