The reason for wanting only part of the state is that we will use NgRx in a context where there will be many components that only care about rendering a little part of the application's full state. For example, we may have a product list component, a product detail component, and so on. For that reason, we need to implement support for getting a slice of state. Thanks to the fact that our store inherits from a BehaviorSubject, implementing a slice of state is child's play:

// NGRX-light/storeVII.js

const Rx = require('rxjs');

function counterReducer(state = 0, action) {
  switch(action.type) {
    case "INCREMENT":
      return state + 1;
    default:
      return state;
  }
}

function productsReducer(state = [], action) {
  switch(action.type) {
    case 'ADD_PRODUCT':
      return [ ...state, Object.assign({}, action.payload) ]
    default:
      return state;
  }
}

class Store extends Rx.BehaviorSubject {
  constructor() {
    super({ counter: 0, products: [] });
    this.dispatcher = new Rx.Subject();
    this.state = {};
    this.dispatcher
      .scan((acc, curr) => ({ ...acc, ...curr }))
      .subscribe(data => this.next(data));
  }

  calcState(state, action) {
    return {
      counter: counterReducer(state.counter, action),
      products: productsReducer(state.products, action)
    }
  } 

  dispatch(action) {
    const newState = this.calcState(this.value, action);
    this.dispatcher.next(newState);
  }

  select(slice) {
    return this.map(state => state[slice]);
  }
}

const store = new Store();
store
  .select('products')
  .subscribe(data => console.log('store using select', data));

// store using select, []
// store using select, [{ id: 1, name: 'Yoda' }]

store.subscribe(data => console.log('store', data));
store.dispatch({ type: 'INCREMENT' });
store.dispatch({ type: 'INCREMENT' });
// store 0
// store 1

store.dispatch({ type: 'ADD_PRODUCT', payload: { id: 1, name: 'Yoda' } });

Should we want a more advanced select method, we can let it take a function instead, like so:

// excerpt from the Store class
select(fn) {
  return this.map(fn);
}

// usage - if there were such a state as 'state.products.list'
store
  .select(state => state.products.list);