Why do I need to clean up side effects in React?
Because if you set up a subscription, timer, or listener and never remove it, it keeps running after the component unmounts. This leaks memory and tries to update state on an unmounted component, causing warnings and bugs.
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More FAQs in How to Clean Up Side Effects in React useEffect
It is the function you return from useEffect. React calls it before the next effect runs and when the component unmounts. It is where you tear down whatever the effect set up, like subscriptions and timers.
Subscribe in the effect and return a function that unsubscribes. React calls that function before the next effect and on unmount, so the subscription is always removed when it is no longer needed.
Use an AbortController. Pass its signal to fetch, and call abort on the controller in the cleanup function. This actually cancels the in-flight request instead of just ignoring its result after unmount.
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