Andrew Kim Andrew Kim - 1 month ago 13x
Javascript Question

ReactJS: Why is the convention to fetch data on componentDidMount?

As per this link,

I've built a todo app that works great. I fetch my data in

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
in the hook.

Why not just fetch the data in
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.


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 (i.e. within node) only componentWillMount gets executed, and therefore could fall over if you are using a browser/DOM dependant feature. The other lifecyle methods (i.e. 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 (i.e. runs on node/browser) and this probably won't be a concern for you.