The store will act as the data source for our state but will also be able tell any listeners when a change to the store happens:

// demo/store.js

import dispatcher from './dispatcher';

function selectIndex(index) {
  store["selectedIndex"] = index;
}

// 1) store registers with dispatcher
dispatcher.register(message => {
  switch (message.type) {
    // 3) message is sent by dispatcher ( that originated from the first view)
    case "SELECT_INDEX":
      selectIndex(message.data);
      // 4) listener, a view, is being notified of the change
      store.emitChange();
      break;
    }
});

class Store {
  constructor() {
    this.listeners = [];
  }

  // 2) listener is added by a view
  addListener(listener) {
    if (!this.listeners["change"]) {
      this.listeners["change"] = [];
    }

    this.listeners["change"].push(listener);
  }

  emitChange() {
    if (this.listeners["change"]) {
      this.listeners["change"].forEach(cb => cb());
    }
  }

  getSelectedItem() {
    return store["selectedIndex"];
  }
}

const store = new Store();
export default store;