Custom Hook

custom_hook.js

import React, { useState, useEffect, useRef } from "react";
import ReactDOM from "react-dom";

const useTimer = () => {
  const [date, setDate] = useState(new Date());
  let interval = useRef();

  useEffect(() => {
    interval.current = setInterval(() => {
      setDate(new Date());
    }, 1000);
    
    return () => {
      clearInterval(interval.current);
    };
  }, []);

  return date.toLocaleTimeString();
};

const App = () => {
  const time = useTimer();

  return <div>{time}</div>;
};

ReactDOM.render(<App />, document.getElementById("root"));