<div id="app"></div>

import React, { useEffect, useRef, useState } from 'https://cdn.skypack.dev/react'
import { render } from 'https://cdn.skypack.dev/react-dom'

const Time = () => {
  const [time, setTime] = useState(Date.now())
  const timer = useRef(null)
  useEffect(() => {
    timer.current = setInterval(() => setTime(Date.now()), 1000)
    return () => clearInterval(timer.current)
  }, [])
  return <h1>{`Time: ${time}`}</h1>
}

const App = () => <Time/>

render(<App/>, document.getElementById('app'))