title: lifecycle event state and prevState in react.js
body: <p>Below is a simple counter in react. But I have 3 questions. </p>
  
  <ol>
  <li><p>What is state in line 3? It looks like a global variable, it would make sense if it has <code>var</code> or <code>const</code> before it. Is that a lifecycle function/var?</p></li>
  <li><p>Do I have to import Component from react? I remember I did not need do that in v15.</p></li>
  <li><p>Where does prevState come from?</p></li>
  </ol>
  
  <blockquote>
  <pre><code>import React, { Component } from 'react';
  
  class Counter extends Component {
    state = { value: 0 };
  
    increment = () =&gt; {
      this.setState(prevState =&gt; ({
        value: prevState.value + 1
      }));
    };
  
    decrement = () =&gt; {
      this.setState(prevState =&gt; ({
        value: prevState.value - 1
      }));
    };
  
    render() {
      return (
        &lt;div&gt;
          {this.state.value}
          &lt;button onClick={this.increment}&gt;+&lt;/button&gt;
          &lt;button onClick={this.decrement}&gt;-&lt;/button&gt;
        &lt;/div&gt;
      )
    }
  }
  </code></pre>
  </blockquote>