title: React's setState method with prevState argument
body: <p>I'm new to React, just have a question on setState method. Let's say we have a component:</p>
  <pre><code>class MyApp extends React.Component {
  
    state = {
      count: 3
    };
  
    Increment = () =&gt; {
      this.setState((prevState) =&gt; ({
        options: prevState.count + 1)
      }));
    }
  }
  </code></pre>
  <p>so why we have to use <code>prevState</code> in the <code>setState</code> method? why can't we just do:</p>
  <pre><code>this.setState(() =&gt; ({
    options: this.state.count + 1)
  }));
  </code></pre>