portal2.js

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {showModal: false};
    this.mostraModal = this.mostraModal.bind(this);
    this.escondeModal = this.escondeModal.bind(this);
  }

  mostraModal() {
    this.setState({showModal: true});
  }

  escondeModal() {
    this.setState({showModal: false});
  }

  render() {
    const modal = this.state.showModal ? (
      <Modal>
        <div className="modal">
          <div>Olá, eu sou um modal!</div>
          <button onClick={this.escondeModal}>Hide modal</button>
        </div>
      </Modal>
    ) : null;

    return (
      <div>
        <button onClick={this.mostraModal}>Show modal</button>
        {modal}
      </div>
    );
  }
}

ReactDOM.render(<App />, document.getElementById('root'));