How to trigger event on field change in Firestore | Cloud Functions [duplicate]

Question

I am trying to trigger an action only if a specific field in a document changes.

Let's say the following Firestore structure is given:

• events
  • eventID
    • title: "Party"
    • invitations:
      • userID_1
      • userID_2

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.

I am working with TypeScript instead of JavaScript.