title: React this is undefined
body: <p>React <code>this</code> is undefined in promise . Here is my code:</p>
  
  <pre><code>export default class Album extends React.Component {
  
      constructor(props) {
          super(props);
      }
  
      componentDidMount ()  {
          console.log(this.props.route.appState.tracks);  // `this` is working
          axios({
              method: 'get',
              url: '/api/album/' + this.props.params.id + '/' + 'tracks/',
              headers: {
                  'Authorization': 'JWT ' + sessionStorage.getItem('token')
              }
          }).then(function (response) {
              console.log(response.data);
              this.props.route.appState.tracks.concat(response.data); // 'this' isn't working
          }).catch(function (response) {
              console.error(response);
              //sweetAlert("Oops!", response.data, "error");
          })
      }
  </code></pre>
  
  <p>here is the error code:</p>
  
  <pre><code>TypeError: this is undefined
  Stack trace:
  componentDidMount/&lt;@webpack:///./static/apps/components/Album.jsx?:81:17
  </code></pre>