title: What is the exact usage of componentWillUpdate in ReactJS?
body: <p>I am new to this ReactJS and started learning Component Lifecycle. Now i have doubt about the <code>componentWillUpdate</code> and <code>componetDidUpdate</code>. I know that once you send <code>return true</code> from <code>shouldComponentUpdate</code> function, it will call <code>componentWillUpdate</code>. Once the component have updated then <code>componetDidUpdate</code> will call. </p>
  
  <p>My question is are we really doing anything in the <code>componetWillUpdate</code> function? I hope we can't use <code>setState</code> also in that function. What is the exact use of componentWillUpdate function? When it will be useful? </p>