use efffect in react code example

Example 1: react use effect

useEffect(() => {
	
  },[]);

Example 2: componentdidupdate in hooks

const App = props => {
  const didMountRef = useRef(false)
  useEffect(() => {
    if (didMountRef.current) {
      doStuff()
    } else didMountRef.current = true
  }
}