React learning problems encountered in

1. When looking at the code thought of a question

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};
  }
 
  componentDidMount() {
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }
 
  componentWillUnmount() {
    clearInterval(this.timerID);
  }
 
  tick() {
    this.setState({
      date: new Date()
    });
  }
 
  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>现在是 {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}
 
ReactDOM.render(
  <Clock />,
  document.getElementById('example')
);

js where there is a pre-compiled, then react this pre-state components have compiled what is it?