react-pure-component.js

var Timer = React.createClass({
  displayName: 'Timer',
  propTypes: {
    interval: React.PropTypes.number,
    onTick: React.PropTypes.func
  },
  getDefaultProps: function() {
    return {
      interval: 1000
    };
  },
  getInitialState: function() {
    return {
      ticksElapsed: 0
    };
  },
  tick: function() {
    this.setState({
      ticksElapsed: this.state.ticksElapsed + 1
    });
    if (this.props.onTick) {
      this.props.onTick();
    }
  },
  componentDidMount: function() {
    this.interval = setInterval(this.tick, this.props.interval);
  },
  componentWillUnmount: function() {
    clearInterval(this.interval);
  },
  render: function() {
    var txt = 'Seconds Elapsed: ' + this.state.ticksElapsed;
    return React.DOM.div(null, txt);
  }
});

var timer = Timer();
React.renderComponent(timer, mountNode);