class App extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      messageConstructor: 'stateが初期化されました。'
    }
  }

  render() {
    return (
      <div>
        <div>【constructor()】{this.state.messageConstructor}</div>
      </div>
    )
  }
}

ReactDOM.render(<　App />, document.getElementById('react-container'));