Memory Leak or “setState on unmounted component” warning in useEffect

While fetching data in useEffect, I sometimes get this warning: "Can't perform a React state update on an unmounted component. This is a no op, but it indicates a memory leak..." I think it's due to calling setState after the component unmounted.

The warning occurs when asynchronous logic continues after the component unmounts—then updates state unexpectedly. This often happens with API calls or subscriptions in useEffect lacking cleanup logic. Fix ✅: Use a cancel flag or cancel tokens to ensure state updates only when the component is still mounted: js Copy code useEffect(() => { let isMounted = true; api.fetchData().then(data => { if (isMounted) setData(data); }); return () => { isMounted = false }; }, []); For fetch/axios, prefer AbortController or cancel token methods in cleanup.