title: React, state, Why the `count` is not defined?
body: <p>When I  click the `Update', this error shows.</p>
  
  <p>The error :</p>
  
  <blockquote>
    <p><code>app.js?7ac9:18 Uncaught ReferenceError: count is not defined</code></p>
  </blockquote>
  
  <pre><code>import  React, {Compoment} from 'react';
  import ReactDOM from 'react-dom';
  class Counter extends  React.Component {
      constructor(props) {
          super(props);
          this.handleClick = this.handleClick.bind(this);
          this.state = {
              count: 0,
          };
      }
      handleClick(e){
          e.preventDefault();
          this.setState({
              count: count + 1,
          });
      }
      render () {
          return(
              &lt;div&gt;
                  &lt;p&gt;{this.state.count}&lt;/p&gt;
                  &lt;a href="#" onClick={this.handleClick}&gt; Update&lt;/a&gt;
              &lt;/div&gt;
              );
      }
  }
  ReactDOM.render(&lt;Counter /&gt;, document.getElementById('app'))
  </code></pre>
  
  <p>I have defined the count with <code>this.state</code> in the constructor. Do you write the good code in the answer?  </p>