title: Angular trackBy returning the index in *ngFor vs item id
body: <p>I have a huge ngFor, changing frequently with asynchronous data. TrackBy allows to only refresh the changing part,  and I really feel the difference when I add it.
  It seems clear to me what is the benefit to have a trackBy returning a unique ID, for example. 
  But I sometime see samples returning the current index.  </p>
  
  <pre><code>public trackByFn(index, item) { return index }
  </code></pre>
  
  <p>In my table, I don't notice any differences if I directly return 'index' or 'item.id'. Both seems optimize the render (but I maybe didn't catch some bugged border cases).</p>
  
  <p>So, can someone explain me what exactly happen when I return the index? </p>