storage-listener.js

export function createStorageListener(store) {
  return event => {
    // the storage event tells you which value changed
    const {action} = JSON.parse(event.newValue);
    
    store.dispatch(action);
  }
}

// when you setup the app
const store = initStore(/*...*/);

if (/* check if this is a 'follower' */) {
  window.addEventListener('storage', createStorageListener(store));
}