ReactJS：为什么约定在componentDidMount上获取数据？

Question

As per this link https://facebook.github.io/react/tips/initial-ajax.html ,

I've built a todo app that works great. I fetch my data in componentDidMount hook like they say to. I just had a question on the why.

Why do we fetch data after the components mounted? Seems to me it just ends up re-rendering once the data gets set in setState() in the hook.

Why not just fetch the data in constructor() and set the initial state with data? Wouldn't that be just the 1 render? Seems less expensive even if its only partially re-rendering because of the virtual DOM.

Answer 1

The fetch will generally be executing asynchronously so it's always going to cause a second render despite it being within componentWillMount or componentDidMount .

I believe they are recommending to put it within the componentDidMount as you may be using a browser dependant implementation of sorts. When executing/rendering components on the server (ie within node) only componentWillMount gets executed, and therefore could fall over if you are using a browser/DOM dependant feature. The other lifecyle methods (ie componentDidMount ) don't get executed on the server, so hence the safety and recommendation I think. Although in general I have found it more useful when needing to do DOM access. Use a universal ajax library (ie runs on node/browser) and this probably won't be a concern for you.