<div id="app"></div>
body {
  margin: 10px;
}

class App extends React.Component {
  
  render() {
    return (
      <button onClick={this.btnClick}>Click Me</button>
    );
  }
  
  btnClick() {
    alert('Clicked')
  }
}

ReactDOM.render(<App />, document.getElementById('app'))
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External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha.6/css/bootstrap.min.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/react/15.6.1/react.js
  2. https://cdnjs.cloudflare.com/ajax/libs/react/15.6.1/react-dom.js