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class Timer extends React.Component {
  constructor(props) {
    super(props);
    this.state = { seconds: 0 };
  }

  tick() {
    this.setState(state => ({
      seconds: state.seconds + 1
    }));
  }

  componentDidMount() {
    this.interval = setInterval(() => this.tick(), 1000);
  }

  componentWillUnmount() {
    clearInterval(this.interval);
  }

  render() {
    return (
      <div>
        Seconds: {this.state.seconds}
      </div>
    );
  }
}

ReactDOM.render(
  <Timer />,
  document.getElementById('example')
);

 <head>
    <script src='https://unpkg.com/react/umd/react.production.min.js'>
    </script>
    <script src='https://unpkg.com/react-dom/umd/react-dom.production.min.js'>
    </script>
    <script src='https://cdnjs.cloudflare.com/ajax/libs/antd/3.0.0-alpha.9/antd.js'>
    </script>
  </head>
  <body>
    <Timer id="example"/>
    
  </body>