const WithTimer = {
  getInitialState() {
    return { ticks: 0 };
  },
  
  componentDidMount() {
    this._withTimer__clock = setInterval(() => {
      this.setState({ ticks: this.state.ticks + 1 });
    }, 1000);
  },
  
  componentWillUnmount() {
    clearInterval(this._withTimer__clock);
  },    
};

const App = React.createClass({  
  getInitialState() { return {} },
  
  mixins: [WithTimer],
  
  render() {
    return (
      <div>
        <p>Hello World {this.state.ticks} </p>
      </div>
    );
  }  
});

ReactDOM.render(<App />, document.querySelector('main'));