How do you clean up side effects in useEffect?

To clean up, return a function from the effect callback. This cleanup function is called when the component unmounts or before the effect re-runs.

For example: When using setInterval we need to clear it or else if will keep running even after the component is unmount

useEffect(() => {
  const timer = setInterval(() => {
    console.log('Timer running');
  }, 1000);
  return () => clearInterval(timer); // Cleanup
}, []);