how does the useEffect hook work? code example

Example 1: useeffect

function App() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    longResolve().then(() => {
      alert(count);
    });
  }, []);

  return (
    <div>
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        Count: {count}
      </button>
    </div>
  );
}

Example 2: useEffectOnce

import {useEffectOnce} from 'react-use';

const Demo = () => {
  useEffectOnce(() => {
    console.log('Running effect once on mount')
    
    return () => {
      console.log('Running clean-up of effect on unmount')
    }
  });

  return null;
};