useEffect

App.js

import React, { useState, useEffect } from "react";

function App() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    const interval = setInterval(() => setCount(count + 1), 1000);

    return () => {
      clearInterval(interval)
    }
  }, [count]);

  return (
    <div>
      <h1>{count} segundos</h1>
    </div>
  );
}

export default App;