useeffect

// Always run
useEffect( () => {
  // code to run
} );

// As componentDidMount (run once)
useEffect( () => {
  // code to run
}, [] );

// Run only when attr changes
useEffect( () => {
  // code to run
}, [ attr ] );

// Run only when component updates (componentDidUpdate)
const didMountRef = useRef( false );
  useEffect( () => {
    if (didMountRef.current) {
      // code to run
    } else didMountRef.current = true
  }