How React's useEffect Cleanup Functions Prevent Memory Leaks and Silent Bugs

React's useEffect hook is widely used for side effects like data fetching, timers, and event listeners, but omitting or incorrectly writing its cleanup function can cause memory leaks, duplicate handlers, and errors on unmounted components. The cleanup function, returned from useEffect, runs before the effect re-executes when dependencies change and again when the component unmounts. Common scenarios requiring cleanup include removing event listeners, clearing intervals, unsubscribing from sockets, and aborting fetch requests using AbortController. React's Strict Mode further exposes cleanup bugs by intentionally mounting, cleaning up, and remounting effects during development. Developers are advised to ensure every opened resource has a corresponding teardown path and that the dependency array is accurate to avoid stale closure issues.