function createStore(reducer, initState, enhancer) {
  if (typeof initState == "function" && enhancer == null) {
    enhancer = initState;
    initState = undefined;
  }

  if (enhancer !== undefined) {
    return enhancer(createStore)(reducer, initState);
  }

  let currentReducer = reducer;
  let currentState = initState;
  let currentListeners = [];
  let nextListeners = currentListeners;

  function dispatch(action) {
    currentState = currentReducer(currentState, action);

    if (currentListeners !== nextListeners) {
      currentListeners = nextListeners;
    }
    for (let i = 0; i < currentListeners.length; i++) {
      const fn = currentListeners[i];
      fn();
    }
  }

  function replaceReducer(nextReducer) {
    currentReducer = nextReducer;
  }

  function subscribe(callback) {
    if (nextListeners === currentListeners) {
      nextListeners = currentListeners.slice();
    }
    nextListeners.push(callback);
    return function unsubscribe() {
      if (nextListeners === currentListeners) {
        nextListeners = currentListeners.slice();
      }
      if (nextListeners.indexOf(callback) > -1) {
        nextListeners.splice(nextListeners.indexOf(callback), 1);
      }
    };
  }

  function getState() {
    return currentState;
  }

  let store = {
    subscribe,
    getState,
    dispatch,
    replaceReducer
  };

  store.dispatch({
    type: "INIT"
  });

  return store;
}

function applyMiddleware(...middlewares) {
  return createStore => (...args) => {
    const store = createStore(...args);
    let dispatch;

    const middlewareAPI = {
      getState: store.getState,
      dispatch: (...args) => dispatch(...args)
    };

    const chain = middlewares.map(middleware => middleware(middlewareAPI));

    dispatch = compose(...chain)(store.dispatch);

    return {
      ...store,
      dispatch
    };
  };
}

function compose(...fns) {
  if (fns.length === 0) {
    return arg => arg;
  }
  if (fns.length === 1) {
    return fns[0];
  }
  return fns.reduce((a, b) => (...args) => a(b(...args)));
}