redux-simple-store.js

// reducer.js
let initialState = 0;

function reducer (state=initialState, action) {
  switch(action.type){
    case 'INCREMENT_COUNT':	
      return state + 1;
    default:
      return state
  }
}

// store.js
function createStore(reducer, localState){
  let state = localState;
  let subscribers = [];
  
  function getState(){
    return state;
  }
  
  function subscribe(cb){
    subscribers.push(cb);
  }
  
  function dispatch(action){
    state = reducer(state, action);
    subscribers.forEach((sub)=>sub());
  }
  
 return {getState, subscribe, dispatch}

}

let reducer = require(./reducer);
let store = createStore(reducer, 0);
store.subscribe(()=>console.log(`current state ${store.getState()}`));
console.log(store.getState());  // logs out 0
store.dispatch({type: 'INCREMENT_COUNT'});  // logs out 1
store.dispatch({type: 'INCREMENT_COUNT'});  // logs out 2
store.dispatch({type: 'INCREMENT_COUNT'}); // logs out 3