title: setState callback is not firing after state change
body: <p>I have a function that I need executing after I have set the state of something. I'm using setStates callback function like so to call it:</p>
  
  <pre><code>setState({
      myState:myState
  },secondFunction())
  
  secondFunction = () =&gt; {
      console.log(this.state.myState);
  }
  </code></pre>
  
  <p>When my callback function has the brackets: <code>()</code> the console will log undefined or nothing but when I remove the brackets from the call back like so it seems to work:</p>
  
  <pre><code>setState({
      myState:myState
  },secondFunction)
  </code></pre>
  
  <p>I'm new to React so if someone could explain to me why having the brackets makes a different in the order in which the function is called that would be very helpful!</p>