const counter = (state = 0, action) => {

  switch(action.type) {

    case 'INCREMENT':

      return state + 1;

    case 'DECREMENT':

      return state - 1;

    default :

      return state;

  }

}

​

const createStore = (reducer) => {

  let state;

  let listeners = [];

  const getState = () => state;

  const dispatch = (action) => {

    state = reducer(state, action);

    listeners.forEach((listener)=>{

      listener();

    });

  };

  const subscribe = (listener) => {

    listeners.push(listener);

    return ()=>{

      listeners = listeners.filter(l => l !== listeners)

    }

  };

  dsipatch({});

  return {getState, dispatch, subscribe};

}

​

const render = () => {

  document.body.innerText = store.getState();

}

​

const { createStore } = Redux;

const store = createStore(counter);

​

store.subscribe(render);

render();

​

document.addEventListener('click', () => {

  store.dispatch({type:'INCREMENT'});

});