React Base Fiddle (JSX)

Starting point for creating JSFiddles with React. This uses React with Addons.

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.24/browser.js"></script>
<script src="https://fb.me/react-with-addons-0.14.6.js"></script>
<script src="https://fb.me/react-dom-0.14.6.js"></script>
<script src="https://facebook.github.io/react/js/jsfiddle-integration-babel.js"></script>

<div id="container">
<p>
jnknwqkjnkj<br>
jhiwhiw (this is 36px height)
</p>
    <!-- This element's contents will be replaced with your component. -->
</div>

JavaScript 1.7

var DivSize = React.createClass({
	getInitialState() {
  	return { state: 0 };
  },
  
  componentDidMount() {
    const height = document.getElementById('container').clientHeight;
    this.setState({ height });
  },

  render: function() {
    return (
	    <div className="test">
      	Size: <b>{this.state.height}px</b> but it should be 18px after the render
      </div>
    );
  }
});

ReactDOM.render(
  <DivSize />,
  document.getElementById('container')
);