redux-essentials-0.js

const actionTypes = {
  ACTION_TYPE_TEST: 'ACTION_TYPE_TEST'
}

class Store {
  constructor(initialState = { testValue: 0 }) {
    this.state = initialState;
    this.subscribers = [];
  }

  dispatch(action) {
    this.state = reducer(this.state, action);
    this.notifySubscribers();
    return action;
  }

  notifySubscribers() {
    this.subscribers.forEach(subscriber => {
      subscriber(this.state);
    });
  }

  subscribe(fn) {
    this.subscribers.push(fn);
  }

  getState() {
    return this.state;
  }
}

const reducer = (state, { type, payload }) => {
  switch (type) {
    case actionTypes.ACTION_TYPE_TEST: {
      return {
        ...state,
        testValue: payload
      }
    }
    default: {
      return state;
    }
  }
}

const myStore = new Store(); // 0

myStore.subscribe(newState => console.warn(newState));

console.warn(myStore.getState().testValue);

myStore.dispatch({ type: actionTypes.ACTION_TYPE_TEST, payload: 42 });

console.warn(myStore.getState().testValue); // 42