title: react-native. Default parameter type ?
body: <p>I am learning React-native. </p>
  
  <p>I saw a following code on <a href="https://facebook.github.io/react-native/docs/listview.html#content" rel="nofollow">ListView sample page of facebook.</a></p>
  
  <pre><code>_renderRow: function(rowData: string, sectionID: number, rowID: number) {
  ....
  </code></pre>
  
  <p>By the way, i have never seen this syntax : <strong><em>'rowData:string'</em></strong></p>
  
  <p>what is a semicolon(:)? </p>
  
  <p>Is this syntax for parameter type checking?</p>
  
  <p>Is this ES6 syntax? </p>