SOURCE

const counter = (state = 0, action) => {
  switch(action.type){
    case 'INCREMENT':
       return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
}

// const { createStore } = Redux;
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 !== listener);
    };
  };
  
  dispatch({});
  
  return { getState, dispatch, subscribe };
}

const store = createStore(counter);

console.log(store.getState());

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

const render = () => {
  document.body.innerText = store.getState();
};

store.subscribe(render);
render();
document.addEventListener('click', () => {
  store.dispatch({type: 'INCREMENT'});
})