redux createStore from scratch

createStore.js

// https://egghead.io/lessons/react-redux-implementing-store-from-scratch

const createStore = (reducer) => {
  let state;
  let listeners = [];
  
  const getState = () => state;

  const dispatch = (action) => {
    // calculate new state
    state = reducer(state, action)
    // notify subscribers know of state change
    listeners.forEach(listener => listener());
  }
  
  const subscribe = (listener) => {
    // keep list of subscribers
    listeners.push(listener);
    // unsubscribe by calling again
    return () => {
      listeners = listeners.filter(l => l !== listener);
    }
  }

  // init state
  dispatch({});
  
  return { getState, dispatch, subscribe };
};