title: How does AngularJS ng-repeat track items by default?
body: <p>I understand that you can use <code>track by expr</code> to specify how items are keyed in a <code>ng-repeat</code> directive.</p>
  
  <p>But without <code>track by</code>, how does AngularJS identify items?</p>
  
  <p>Could I control how objects are tracked by altering the object, without using <code>track by</code>, for example?</p>
  
  <p>From the docs: </p>
  
  <blockquote>
    <p>If no tracking function is specified the ng-repeat associates elements by identity in the collection.</p>
  </blockquote>
  
  <p>But this is not clear. What does "identity in the collection" mean?</p>