useLayoutEffect

The problem

You measure a node and position a tooltip in useEffect. The browser has already painted the tooltip at 0,0. Users see a flash, then a jump. SSR frameworks warn because layout effects do not run on the server.

The pattern that fixes it

useLayoutEffect runs after DOM updates and before the browser paints. The tooltip is placed in the same frame as the new markup. Use it for measure/mutate; keep network and subscriptions in useEffect.

Keep learning

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• Next: Fetch races
• Related: Moving state down
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• Related: Render props
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