Optimize usePrevious

[FleetAdmiralJakob]

Following this YT video: https://www.youtube.com/watch?v=B-Xb_8n5wRg the usePrevious hook probably should not use useRef but instead something closer to that implementation: https://github.com/uidotdev/usehooks/blob/90fbbb4cc085e74e50c36a62a5759a40c62bb98e/index.js#L1017-L1027

Other libraries discuss this too as you can see here: streamich/react-use#2605

[childrentime]

Thank you very much for pointing this out. This is an interesting question, and I quickly watched the video and it seems to be related to the concurrent mode of react18. I know about concurrent mode, but I am not very familiar with this mode.

[childrentime]

It's late at night in my timezone right now, you can submit a PR or I'll change it tomorrow.

[childrentime]

By the way, I also have an interesting example of concurrent mode here: https://reactarium.vercel.app/#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

[childrentime]

https://reactarium.vercel.app/#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 No. I was mistaken, it has nothing to do with concurrent mode and behaves the same in react 17.

[childrentime]

So the traditional usePrevious hook worked because while useRef was updated, React didn't re-render - it just appeared to work correctly since the ref update wasn't triggering a new render cycle. I found this implementation quite elegant too. It demonstrates that you can achieve the same result without setState by using useRef: https://github.com/alibaba/hooks/blob/master/packages/hooks/src/usePrevious/index.ts

[FleetAdmiralJakob]

Interesting

So the traditional usePrevious hook worked because while useRef was updated, React didn't re-render - it just appeared to work correctly since the ref update wasn't triggering a new render cycle. I found this implementation quite elegant too. It demonstrates that you can achieve the same result without setState by using useRef: alibaba/hooks@master/packages/hooks/src/usePrevious/index.ts

While this looks nice I think it violates the rule outlined in the pitfall section too, right?

https://react.dev/reference/react/useRef

[FleetAdmiralJakob]

btw. alibaba has the same discussion 😂: alibaba/hooks#2162

[childrentime]

You're right. I'll migrate to a pattern that doesn't rely on refs👍👍👍.

[prlnd]

It's allowed to write ref.current conditionally: https://react.dev/reference/react/useRef#avoiding-recreating-the-ref-contents

Edit: nvm, it's only allowed for initialization.

[childrentime]

Yes. This is typically performancfe improve method. Often used with Intersetion Observer initialization or other thing

[childrentime]

I'm thinking about the implementation of useLatest - it writes to the ref during render. However, it seems difficult to simply migrate this operation to useEffect instead?: https://stackoverflow.com/questions/68512910/concurrent-safe-version-of-uselatest-in-react