useEffect cleanup example

useeffect_example.js

const [showLoading, setShowLoading] = useState(false)

 useEffect(
    () => {
      let timer1 = setTimeout(() => setShowLoading(true), 1000)

      // this will clear Timeout when component unmont like in willComponentUnmount
      return () => {
        clearTimeout(timer1)
      }
    },
    [] //useEffect will run only one time
       //if you pass a value to array, like this [data] than clearTimeout 
       //will run every time this value changes (useEffect re-run)
  )