Counter.js

export const Counter = ({ initialValue, onChange }) => {
  const [count, setCount] = useState(initialValue)

  useEffect(() => onChange(count), [count])

  const incrementCounter = () => setCount(count + 1)

  return (
    <div>
      Count: {count}
      <button onClick={incrementCounter}>+</button>
    </div>
  )
}