index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
    <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
    <script src="https://unpkg.com/@babel/standalone/babel.min.js"></script>
    <title>Document</title>
</head>
<body>
    <div id="app"></div>
    <script type="text/babel">
        class App extends React.Component{ 
            state = {
                name:'Rohan',
                age:21
            }
            whenchange =(e)=>{
                this.setState({
                    name:e.target.value
                });
            }
            whensubmit =(e)=>{
                e.preventDefault();
                console.log('form submitted',this.state.name);
            }
            render() { 
                return ( 
                    <div>
                        <p>My name is { this.state.name } and I am {this.state.age} years old.</p>
                        <form onSubmit={this.whensubmit} >
                            <input type="text" onChange={this.whenchange} />
                            <button>update</button>    
                        </form>   
                    </div> 
                    )
                }
            } 
        ReactDOM.render(< App /> , document.getElementById("app"));
    </script>
</body>
</html>