title: How to trigger event on field change in Firestore | Cloud Functions
body: <p>I am trying to trigger an action only if a specific field in a document changes.</p>
  <p>Let's say the following Firestore structure is given:</p>
  <ul>
  <li>events
  <ul>
  <li>eventID
  <ul>
  <li>title: &quot;Party&quot;</li>
  <li>invitations:
  <ul>
  <li>userID_1</li>
  <li>userID_2</li>
  </ul>
  </li>
  </ul>
  </li>
  </ul>
  </li>
  </ul>
  <p>I now would like to trigger an action if the invitations array is being updated.
  So far I can only find that triggers can be applied to a document (eventID in this case), but not to a field. Is there no possibility to have a trigger on a specific field.</p>
  <p>I am working with TypeScript instead of JavaScript.</p>