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useEffect vs useLayoutEffect

November 26, 2018


The simple rules for when to use each.

Both of these can be used to do basically the same thing, but they have slightly different use cases. So here are some rules for you to consider when deciding which React Hook to use.

useEffect

99% of the time this is what you want to use. When hooks are stable and if you refactor any of your class components to use hooks, you'll likely move any code from componentDidMount, componentDidUpdate, and componentWillUnmount to useEffect.

The one catch is that this runs after react renders your component and ensures that your effect callback does not block browser painting. This differs from the behavior in class components where componentDidMount and componentDidUpdate run synchronously after rendering. It's more performant this way and most of the time this is what you want.

However, if your effect is mutating the DOM (via a DOM node ref) and the DOM mutation will change the appearance of the DOM node between the time that it is rendered and your effect mutates it, then you don't want to use useEffect. You'll want to use useLayoutEffect. Otherwise the user could see a flicker when your DOM mutations take effect. This is pretty much the only time you want to avoid useEffect and use useLayoutEffect instead.

useLayoutEffect

This runs synchronously immediately after React has performed all DOM mutations. This can be useful if you need to make DOM measurements (like getting the scroll position or other styles for an element) and then make DOM mutations or trigger a synchronous re-render by updating state.

As far as scheduling, this works the same way as componentDidMount and componentDidUpdate. Your code runs immediately after the DOM has been updated, but before the browser has had a chance to "paint" those changes (the user doesn't actually see the updates until after the browser has repainted).

Summary

  • useLayoutEffect: If you need to mutate the DOM and/or DO need to perform measurements
  • useEffect: If you don't need to interact with the DOM at all or your DOM changes are unobservable (seriously, most of the time you should use this).

Conclusion

I am extremely excited about React's upcoming hooks feature. I think it's going to make React much easier to learn and use.

Learn more about React from me: Hooks & Suspense Playlist on Egghead.io — A free 35 minute list of videos demoing how to use the new React Hooks and Suspense features. (Note: these features are still pretty alpha and likely to change).

Things to not miss:

  • useHooks.com — One new React Hook recipe every day. Really cool resource by Gabe Ragland.
  • fresh-concurrent-react by Shawn Wang — an as up-to-date-as-possible resource about the upcoming features in concurrent react! Really helpful if you want to play around with Suspense and Concurrent React.
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