react-learning/13-callback.html at main · YasminFerrariDias/react-learning · GitHub

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<!-- useCallback para memorizar funções e evitar recriação desnecessária a cada render. -->
<!doctype html>
<html lang="pt-br">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Rocketseat - React.js - Fundamentos</title>
  </head>
  <body>
    <div id="root"></div>

    <script src="https://unpkg.com/react@18/umd/react.development.js"></script>
    <script src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>
    <script src="https://unpkg.com/@babel/standalone/babel.min.js"></script>

    <script type="text/babel">
      function App() {
        const [is12Hours, setIs12Hours] = React.useState(false);
        const [time, setTime] = React.useState("");

        const updateTime = React.useCallback(() => {
          const now = new Date(Date.now());
          const formattedTime = now.toLocaleTimeString("pt-BR", {
            hour: "2-digit",
            minute: "2-digit",
            second: "2-digit",
            hour12: is12Hours,
          });

          setTime(formattedTime);
        });

        React.useEffect(() => {
          updateTime();

          const timer = setInterval(() => {
            updateTime()
          }, 1000);

          return () => clearInterval(timer);
        }, [updateTime]);

        return (
          <main>
            <div>🕒 {time}</div>

            <button onClick={() => setIs12Hours(!is12Hours)} >Alterar para formato {is12Hours ? "24h" : "12h"}</button>
          </main>
        );
      }

      ReactDOM.createRoot(document.getElementById("root")).render(<App />);
    </script>
  </body>
</html>