之前学习React 期间，就看到很多地方提到 side effect 副作用
react what is side effect
“What is a side-effect?
The natural Redux flow is this: some action is dispatched, and as a consequence, some state is changed.
If all your Redux app could do was just to change state as the user played with it, though, would your app be particularly useful? Probably not.
Most apps need to reach out to the outside world to be useful — whether by talking to the server, accessing local storage, recording analytics events, or something else entirely.
That process of calling into the real world is what side-effects are. They are a way of bridging the pure Redux world with the outside world.
Common features of side-effects
Side-effects can happen in response to Redux actions. For example, when a user clicks “Save,” you may want to fire off an AJAX request.
Side-effects may dispatch Redux actions. Like when the save process finishes successfully, you may want to dispatch SAVE_SUCCEEDED; or when it failed, SAVE_FAILED.
They also may not dispatch anything. Some side-effects don’t need to dispatch anything: if you are doing analytics tracking based on Redux actions, you would track things in response to Redux actions, but you will not dispatch anything.”
“React+Flux more complicated processes involving coordination of action creators and side-effects are hard to handle. ”
“side effects (i.e. asynchronous things like data fetching and impure things like accessing the browser cache) ”
side effect副作用是：和真实的世界进行交互的方式，就叫做 side effect。
用户点击了 保存 按钮，然后会异步地调用服务器的接口去保存数据。