React Time Counter

HTML

<script src="//fb.me/react-with-addons-0.14.0.js"></script>
<script src="//fb.me/react-dom-0.14.0.js"></script>
<div id="container"></div>

Babel + JSX

"use strict";

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

ReactDOM.render(<Timer />, document.getElementById("container"));