React Js Fullscreen Api

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8" />
    <script src="https://unpkg.com/react@17.0.2/umd/react.development.js"></script>
    <script src="https://unpkg.com/react-dom@17.0.2/umd/react-dom.development.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.26.0/babel.min.js"></script>
</head>
<body>
    <div id="app"></div>
    <script type="text/babel">
        const { useState, useEffect } = React
        function App() {
            const [fullscreen, setFullscreen] = useState(false);

useEffect(() => {
                const updateFullScreenState = () => {
                    setFullscreen(Boolean(document.fullscreenElement));
                };

updateFullScreenState();
                document.addEventListener('fullscreenchange', updateFullScreenState);

return () => {
                    document.removeEventListener('fullscreenchange', updateFullScreenState);
                };
            }, []);

return (
                <div className='container'>
                    <h3>React Toggle Fullscreen</h3>
                    {fullscreen ? (
                        <p>App is in fullscreen mode</p>
                    ) : (
                        <p>App is not in fullscreen mode</p>
                    )}
                    {!fullscreen ? (
                        <button onClick={toggleFullScreen}>Fullscreen</button>
                    ) : (
                        <button onClick={toggleFullScreen}>Exit Fullscreen</button>
                    )}
                </div>
            );
        };
        ReactDOM.render(<App />, document.getElementById("app"));
    </script>
    <style>
        .container {
            width: 600px;
            margin: 0 auto;
            box-shadow: 0 1px 2px 0 rgba(0, 0, 0, 0.12), 0 2px 4px 0 rgba(0, 0, 0, 0.24);
            padding: 20px;
            text-align: center;
        }

h3 {
            font-size: 24px;
            margin-bottom: 20px;
        }

p {
            font-size: 18px;
            margin-bottom: 10px;
        }

button {
            padding: 10px 20px;
            font-size: 16px;
            background-color: #4caf50;
            color: white;
            border: none;
            border-radius: 4px;
            cursor: pointer;
        }

button:hover {
            background-color: #45a049;
        }

button:focus {
            outline: none;
        }

button:active {
            background-color: #3e8e41;
        }
    </style>

</body>

</html>