tweetbox.jsx

var TweetBox = React.createClass({
  getInitialState(){
    return {remaining: 140}
  },
  handleKeyUp() {
    var text = this.refs.tweettweet.getDOMNode().value;
    this.setState({remaining: (140 - text.length)});
  },
  render() {
   return <div className="tweetbox">
      { this.state.remaining} Left
       <br/>
       <textarea onKeyUp={this.handleKeyUp} ref="tweettweet"></textarea>
       </div>
    }
});

React.render(<TweetBox></TweetBox>, document.body);