React Js String Charat Method

<!DOCTYPE html>
<html>

<body>
    <div id="app"></div>
    <script type="text/babel">
        function App() {
            const str = "Hello, world!";
            const newStr = str.replace(str.charAt(0), "J");

return (
                <div className='container'>
                    <h3>React Js charAt() Method</h3>
                    <p>Old String: {str}</p>
                    <p>New String: {newStr}</p>

</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;
        }

h3 {
            color: #333;
            font-size: 24px;
            margin-bottom: 10px;
        }

p {
            color: #555;
            font-size: 18px;
            margin-bottom: 8px;
        }

p:last-child {
            margin-bottom: 0;
        }
    </style>
</body>

</html>