useIsMounted.js

import { useState, useEffect } from 'react'

// Use this hook to quickly read whether a component is being rendered server
// side or client-side. This can be helpful when building components that are 
// intended to work without client-side JavaScript. 

// Use like:
// const isMounted = useIsMounted()

const useIsMounted = () => {
  const [ isMounted, setIsMounted ] = useState(false)
  useEffect(() => setIsMounted(true), [])

  return isMounted
}