JS Bin// source http://jsbin.com/cequnu

Redux 7: Implementing store from cratch final

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 !== listener);
    }
  };
  // to get reducer to return initial value
  dispatch({});
  
  return { getState, dispatch, subscribe};
}
const store = createStore(counter);

const render = () => {
  document.body.innerText = store.getState();
}
// Needed to show initial state 0
store.subscribe(render);
// Needed to show initial state 0
render();

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