title: Referencing local variable without `this` in function component
body: <p>I want to refer to the variable in the function component,
  I think normally I use <code>this</code> to refer to the local variable.</p>
  <p>However, in the function component, I can't use <code>this</code>.</p>
  <p>Maybe, I am still confused by the function component though...</p>
  <pre><code>export default function TrackPage(props) {
      const classes = useStyles();
  
      useEffect(() =&gt; {
         var playlist = 1;
      }
      function handleToolBarClick(){
          this.playlist // this is empty.
      }
  }
  </code></pre>