title: How to hide an HTML element if a value is present in an array
body: <p>My controller retrieves an Observable (a FirebaseObjectObservable actually) that I consume in my view with the async pipe.</p>
  
  <p>My object as a list of id called 'joiners'.</p>
  
  <p>I would like to hide a 'join' button whenever the current user id 'cuid' is present in the 'joiners' array.</p>
  
  <p>I tried something like this:</p>
  
  <pre><code>[hidden]="(event$ | async)?.joiners.indexOf(cuid) &gt; -1"
  </code></pre>
  
  <p>But it is not working... any idea?</p>