Redux simple implementation

redux.js

function createStore(reducer, initialState) {
  let state = initialState;
  const listeners = [];
  const subscribe = listener => listeners.push(listener);

  const getState = () => state;
  const dispatch = action => {
    state = reducer(state, action);
    listeners.forEach(l => l());
  };

  return {
    subscribe,
    getState,
    dispatch
  };
}

const messagesReducer = (state, action) => {
  switch (action.type) {
    case "ADD_MESSAGE":
      return { ...state, messages: state.messages.concat(action.message) };
    default:
      return state;
  }
};
const messagesInitState = { messages: [] };
const addMessageAction = message => ({ type: "ADD_MESSAGE", message });

const store = createStore(messagesReducer, messagesInitState);

const listener = () => {
  console.log("Current state: ");
  console.log(store.getState());
};

store.subscribe(listener);

store.dispatch(addMessageAction("Hello Mark!"));
store.dispatch("Wrong action, without 'type' property, nothing happens!");
store.dispatch(addMessageAction("Hello Mark again!"));