demo/07/index.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Hello State</title>
  <script src="https://cdn.staticfile.org/react/16.4.0/umd/react.development.js"></script>
  <script src="https://cdn.staticfile.org/react-dom/16.4.0/umd/react-dom.development.js"></script>
  <script src="https://cdn.staticfile.org/babel-standalone/6.26.0/babel.min.js"></script>
</head>

<body>
  <div id="root"></div>

  <script type="text/babel">
    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, React!</h1>
            <h2>现在是北京时间：{this.state.date.toLocaleTimeString()}</h2>
          </div>
        )
      }
    }

    ReactDOM.render(
      <Clock />,
      document.getElementById('root')
    )
  </script>
</body>

</html>