React Js Get first index of item in array

<!DOCTYPE html>
<html>

<body>
    <div id="app"></div>
    <script type="text/babel">
        const { useState, useEffect } = React;
        function App() {
            const array = [1, 2, 3, 4, 3, 2, 1];
            const element = 3;

const lastIndex = array.lastIndexOf(element);

return (
                <div className='container'>
                    <h3>React Js lastIndexOf() Method</h3>
                    <p>Last index of <span>{element}</span> in the array: <span>{lastIndex}</span></p>
                </div>
            );
        }

ReactDOM.render(<App />, document.getElementById('app'));
    </script>
    <style>
        .container {
            width: 400px;
            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
        }

h3 {
            font-size: 18px;
            margin-bottom: 10px;
        }

.container p {
            font-size: 18px;
            color: #333;
            margin: 0;
        }

.container p span {
            font-weight: bold;
        }
    </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 array = [1, 2, 3, 4, 3, 2, 1];
            const element = 3;
​
            const lastIndex = array.lastIndexOf(element);
​
            return (
                <div className='container'>
                    <h3>React Js lastIndexOf() Method</h3>
                    <p>Last index of <span>{element}</span> in the array: <span>{lastIndex}</span></p>
                </div>
            );
        }
​
        ReactDOM.render(<App />, document.getElementById('app'));
    </script>
    <style>
        .container {
            width: 400px;
            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
        }
​
        h3 {
            font-size: 18px;
            margin-bottom: 10px;
        }
​
        .container p {
            font-size: 18px;
            color: #333;
            margin: 0;
        }
​
        .container p span {
            font-weight: bold;
        }
    </style>
</body>
​
</html>