starting-simple.jsx

import React from 'react';
import { render } from 'react-dom';

class Page extends React.Component {
  state = {
    shouldShowBox: true
  };

  toggleBox = () => {
    this.setState({
      shouldShowBox: !this.state.shouldShowBox
    });
  };

  render () {
    return <div className="page">

      { this.state.shouldShowBox && <div className="box"/>}

      <button
        className="toggle-btn"
        onClick={this.toggleBox}
      >
        toggle
      </button>
    </div>;
  }
}

render(<Page/>, document.querySelector('#container'));