title: What is "this" referring to when binding event callback function in React component with "null" argument?
body: <p>In <a href="https://css-tricks.com/learning-react-container-components/#article-header-id-7" rel="nofollow noreferrer">this React tutorial</a> a button click event callback function inside a React Presentational Component is bound to <code>null</code> explicitely:</p>
  
  <pre><code>&lt;button onClick={_this.props.toggleActive.bind(null, user.id)}&gt;Toggle Active&lt;/button&gt;
  </code></pre>
  
  <p>However, inside the callback function the <code>this</code> keyword is used to access the React Container Component's state:</p>
  
  <pre><code>toggleActive: function(userId) {
     ...
     var newState = Object.assign({}, this.state)
     ...
  }
  </code></pre>
  
  <p>I do not understand why this example works when <code>this</code> should equal <code>null</code> or, in non-strict mode, the global <code>window</code> object. Can anybody tell me why the example is working anyway?</p>
  
  <p>The corresponding Codepen can be found <a href="http://codepen.io/bradwestfall/pen/oxBGRa" rel="nofollow noreferrer">here</a>.</p>