Creating a React js Countdown Timer

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8" />
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.development.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/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 [timeLeft, setTimeLeft] = useState(18000); // seconds (5 hours)

useEffect(() => {
                const timer = setInterval(() => {
                    setTimeLeft(prevTime => prevTime - 1);
                    if (timeLeft === 0) {
                        clearInterval(timer);
                        // countdown finished
                    }
                }, 1000);

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

const hours = Math.floor(timeLeft / 3600).toString().padStart(2, '0');
            const minutes = Math.floor((timeLeft % 3600) / 60).toString().padStart(2, '0');
            const seconds = (timeLeft % 60).toString().padStart(2, '0');

const handleReset = () => {
                setTimeLeft(18000); // Reset time to 5 hours
            };

return (
                <div className='container'>
                    <h2>React Js Countdown Timer</h2>
                    <div className="timer">
                        <span className="time">{hours}</span> : <span className="time">{minutes}</span> : <span className="time">{seconds}</span>
                    </div>
                    <button onClick={handleReset}>Reset</button>
                </div>
            );
        }

ReactDOM.render(<App />, document.getElementById('app'));
    </script>
    <style>
        .container {
            text-align: center;
            box-shadow: 0 1px 2px 0 rgba(0, 0, 0, 0.12), 0 2px 4px 0 rgba(0, 0, 0, 0.24);
            width: 600px;
            margin: 0 auto;
            padding: 20px;
        }

h2 {
            font-size: 24px;
            margin-bottom: 20px;
        }

.timer {
            display: flex;
            align-items: center;
            justify-content: center;
            font-size: 36px;
            margin-bottom: 20px;
        }

.time {
            padding: 5px;
            background-color: #333;
            color: #fff;
            border-radius: 4px;
        }

button {
            padding: 10px 20px;
            background-color: #007bff;
            color: #fff;
            border: none;
            border-radius: 4px;
            font-size: 16px;
            cursor: pointer;
            transition: background-color 0.3s ease;
        }

button:hover {
            background-color: #0056b3;
        }
    </style>
</body>

</html>

 
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8" />
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.development.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/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 [timeLeft, setTimeLeft] = useState(18000); // seconds (5 hours)
​
            useEffect(() => {
                const timer = setInterval(() => {
                    setTimeLeft(prevTime => prevTime - 1);
                    if (timeLeft === 0) {
                        clearInterval(timer);
                        // countdown finished
                    }
                }, 1000);
​
                return () => {
                    clearInterval(timer);
                };
            }, []);
​
            const hours = Math.floor(timeLeft / 3600).toString().padStart(2, '0');
            const minutes = Math.floor((timeLeft % 3600) / 60).toString().padStart(2, '0');
            const seconds = (timeLeft % 60).toString().padStart(2, '0');
​
            const handleReset = () => {
                setTimeLeft(18000); // Reset time to 5 hours
            };
​
            return (
                <div className='container'>
                    <h2>React Js Countdown Timer</h2>
                    <div className="timer">
                        <span className="time">{hours}</span> : <span className="time">{minutes}</span> : <span className="time">{seconds}</span>
                    </div>
                    <button onClick={handleReset}>Reset</button>
                </div>
            );
        }
​
        ReactDOM.render(<App />, document.getElementById('app'));
    </script>
    <style>
        .container {
            text-align: center;
            box-shadow: 0 1px 2px 0 rgba(0, 0, 0, 0.12), 0 2px 4px 0 rgba(0, 0, 0, 0.24);
            width: 600px;
            margin: 0 auto;
            padding: 20px;
        }
​
        h2 {
            font-size: 24px;
            margin-bottom: 20px;
        }
​
        .timer {
            display: flex;
            align-items: center;
            justify-content: center;
            font-size: 36px;
            margin-bottom: 20px;
        }
​
        .time {
            padding: 5px;
            background-color: #333;
            color: #fff;
            border-radius: 4px;
        }
​
        button {
            padding: 10px 20px;
            background-color: #007bff;
            color: #fff;
            border: none;
            border-radius: 4px;
            font-size: 16px;
            cursor: pointer;
            transition: background-color 0.3s ease;
        }
​
        button:hover {
            background-color: #0056b3;
        }
    </style>
</body>
​
</html>