title: Joining property values of objects in an array
body: <p>I have an array of objects. The objects have a property called <code>userName</code>. Is there a way to concatenate the <code>userName</code> values into a comma delimited string? I assume I can use the <code>join</code> function but the only way I can think of takes two steps.</p>
  
  <pre><code>var userNames: string[];
  objectArr.forEach((o) =&gt; { userNames.push(o.userName); });
  var userNamesJoined = userNames.join(",");
  </code></pre>
  
  <p>Is there a way to do it in one line of code?</p>