client.js

import React from "react";
import ReactDOM from "react-dom";
class App extends React.Component {
  
  constructor(){
    super();
    this.state = {text: "click to change"};
  }
  
  ouch(){
    this.setState({test: "woot"});
  }
  
  render() {
    return <div>
      <p>{this.state.text}</p>
      <button onClick={this.ouch}>Yo</button>
    </div>
  }
  
}
var app = document.getElementById('app');
ReactDOM.render(<App />, app);