store.js

const _state = Symbol('previous_state');
const _listeners = Symbol('listeners');

class Store {

  constructor(state = {}) {
    this[_state] = {};
    this[_listeners] = [];
    this.setState(state);
  }

  setState (state = {}) {
    state = Object.assign({}, this[_state], state);
    this[_state] = state;
    this.dispatch(state);
    return this;
  }

  getState () {
    return Object.assign({}, this[_state]);
  }

  dispatch (state) {
    if (!this[_listeners].length) return;
    state = state || this[_state];
    this[_listeners].slice().forEach(listener => listener());
  }

  subscribe (listener) {
    this[_listeners].push(listener);
  }

  unsubscribe (listener = null) {
    if (listener == null) {
      this[_listeners] = [];
    }
    let listeners = this[_listeners];
    const index = listeners.indexOf(listener)
    listeners.splice(index, 1);
  }
}

// Usage

S = new Store({a:10});

let listener = () => {
  console.log('change');
  let state = S.getState();
  console.log(state)
}

S.subscribe(listener);
S.setState({b:20});
S.setState({a:11});
S.unsubscribe(listener);
S.setState({a:12});

subclassable.js

class TimerStore extends Store {
  constructor() {
    super(...arguments);
    this.updateStateOnInterval();
  }

  updateStateOnInterval () {
    this._interval = setInterval(() => {
      let timer = this.getState().timer;
      this.setState({timer: ++timer});
    }, 1000);
  }
}

let ts = new TimerStore({timer: 0});
let listener2 = () => console.log(ts.getState());
ts.subscribe(listener2);