react.dev/_js/examples/timer.js

var TIMER_COMPONENT = "\
var Timer = React.createClass({\n\
  getInitialState: function() {\n\
    return {secondsElapsed: 0};\n\
  },\n\
  tick: function() {\n\
    this.setState({secondsElapsed: this.state.secondsElapsed + 1});\n\
  },\n\
  componentDidMount: function() {\n\
    this.interval = setInterval(this.tick, 1000);\n\
  },\n\
  componentWillUnmount: function() {\n\
    clearInterval(this.interval);\n\
  },\n\
  render: function() {\n\
    return (\n\
      <div>Seconds Elapsed: {this.state.secondsElapsed}</div>\n\
    );\n\
  }\n\
});\n\
\n\
React.render(<Timer />, mountNode);\
";

React.render(
  <ReactPlayground codeText={TIMER_COMPONENT} />,
  document.getElementById('timerExample')
);