title: Is this new ES6 feature?
body: <p>I am learning React Native but do not understand some syntax used in <a href="https://facebook.github.io/react-native/docs/listview.html#content" rel="nofollow">https://facebook.github.io/react-native/docs/listview.html#content</a></p>
  
  <pre><code> _pressData: ({}: {[key: number]: boolean}),
  
  _genRows: function(pressData: {[key: number]: boolean}): Array&lt;string&gt; {
  </code></pre>
  
  <p>I tried to check if it is new ES6 feature but no luck. I would appreciate if someone can give me a reference to that syntax or explain it here. </p>
  
  <p>Thank you.</p>