title: What are the callbacks in ReactTransitionGroup hooks?
body: <p>React.js offers a low level API for animations called <a href="https://facebook.github.io/react/docs/animation.html#low-level-api-reacttransitiongroup" rel="noreferrer">ReactTransitionGroup</a>. In the docs it is stated that for the hooks <code>componentWillAppear</code>, <code>componentWillEnter</code> and <code>componentWillLeave</code>, a callback is passed as an argument.</p>
  
  <p>My question is, what exactly is this callback and how it gets passed to those hooks, the docs aren't saying anything about those callbacks except that the animation is delayed until they get called.</p>