in stories/TransitionGroup.js [94:104]
render() {
const { hide, count } = this.state;
return (
<div>
<button onClick={this.handleClick}>Toggle item</button>
<TransitionGroup timeout={FADE_TIMEOUT}>
{!hide && <Fade key="item">Changing! {count}</Fade>}
</TransitionGroup>
</div>
);
}