subscribe.js

const subscribeMiddleware = () => store => {
  let handlers = [];

  store.subscribe = (cb) => {
    handlers = handlers.concat(cb);
    const removeSubscriber = () => { handlers = handlers.filter(fn => fn !== cb) };
    return removeSubscriber;
  };

  return next => (action) => {
    const previousState = store.getState();
    next(action);
    const nextState = store.getState();

    if (previousState !== nextState) handlers.forEach(cb => cb());
  }
};