demo from https://medium.freecodecamp.org/learn-react-js-in-5-minutes-526472d292f4

index.html

<script src="https://unpkg.com/react@15/dist/react.min.js"> </script><script src="https://unpkg.com/react-dom@15/dist/react-dom.min.js">
</script>
<script src="https://unpkg.com/babel-standalone@6.15.0/babel.min.js"></script>

<div id="root"></div>
<script type="text/babel">
class Hello extends React.Component {
    
    constructor(){
        super();
        this.state = {
            message: "my friend (from state)!"
        };
    }
    updateMessage = () => {
        this.setState({
            message: "my friend (from changed state)!"
        });
    }
    render() {
         return (
           <div>
             <h1>Hello {this.state.message}!</h1>
             <button onClick={this.updateMessage}>Click me!</button>
           </div>   
        )
    }
}
ReactDOM.render(
    <Hello message="my friend" />, 
    document.getElementById("root")
);
</script>