Most simple React component using ES6 class & JSX

es6-component.html

<div id="root"></div>
<script src="https://unpkg.com/react@16.2.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.2.0/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/babel-standalone@6.26.0/babel.js"></script>
<script type="text/babel">
class Button extends React.Component {
  constructor() {
    super(); this.state = { count: 0 }
  }
  updateCount() {
    this.setState((prevState, props) => {
      return { count: prevState.count + 1 } })
  }
  render() {
    return (<button onClick={() => this.updateCount()}>
              Klikattu {this.state.count} kertaa
            </button>) }
}
const rootElement = document.getElementById('root')
const element = <Button />
ReactDOM.render(element, rootElement)
</script>