DocumentChange.newIndex equals -1 for 'added' events

[MieleVL]

• Operating System version: Windows 10
• Browser version: Chrome (latest)
• Firebase SDK version: 8.0.2
• Firebase Product: Firestore

[REQUIRED] Describe the problem

DocumentChange.newIndex === -1 for 'added' events. According to the documentation, this should only be the case for 'removed' events: https://firebase.google.com/docs/reference/js/firebase.firestore.DocumentChange#newindex

Steps to reproduce:

Create a query snapshot listener. When the callback is triggered for the first time, change.newIndex is always -1.

Relevant Code:

db.collection("cities").docChanges().forEach(change => {
  if (change.oldIndex === -1) {
    // Should only be true for added documents.
    // change.newIndex also equals -1, which should not be the case imo. Should be >= 0.
  }

  if (change.newIndex === -1) {
    // Should only be true for removed documents.
    // Currently also true for added documents.
  }
});

[schmidt-sebastian]

@MieleVL Thanks for filing this. We have been changing the internals of our SDK and it looks like this might be a regression. We will try to get this fixed as soon as possible.