Similar to line 553, usersCount may be undefined here. While line 599 uses room.usersCount || 0, the type signature on line 567 declares usersCount as required (not optional). However, IRoom's usersCount field is likely optional in the actual type definition. Consider using the nullish coalescing operator ?? instead of || to handle the case where usersCount is legitimately 0 (though this would be a valid count), or ensure the type signature reflects that usersCount is optional.

There is a potential race condition between checking for owners (lines 654-657) and promoting a new owner (line 664). If another process removes or adds owners between these operations, the logic could fail or produce incorrect results. After promoting a new owner, the code does not verify if the promotion was successful before proceeding with user removal. Consider implementing a transaction or adding verification that a new owner was successfully assigned before removing the old owners.

The promoteOldestByRoomIdExcludingUserIdsToOwner method does not filter for subscriptions that don't already have the 'owner' role. Using $addToSet will not cause an error if the user is already an owner, but it means the "oldest" member promoted might already be an owner. This could lead to unexpected behavior where an existing owner is selected instead of promoting a regular member. Consider adding a filter to exclude users who already have the 'owner' role: 'roles': { $ne: 'owner' } or 'roles': { $not: { $in: ['owner'] } }.

The warning message states "Cannot assign new owner" when remainingMemberSub is null, but the code still proceeds to remove all users regardless (line 676). This could leave a room without any owner. Consider whether this is the intended behavior, or if the removal should be aborted when no suitable member can be promoted to owner. If rooms without owners are acceptable in this scenario, the warning message should clarify this is expected.

When userIds is an empty array, the queries $in: [] and $nin: [] will behave in specific ways: $in: [] matches nothing (returns false), while $nin: [] matches everything (returns true if any owner exists). Consider adding explicit guards at the beginning of these methods to handle empty arrays appropriately and make the behavior explicit. For hasAnyOwnerInUserIds with an empty array, returning false seems correct, but for hasAnyOwnerNotInUserIds with an empty array, the current behavior might be surprising to callers.

The calculation of remaining members count is incorrect when users array contains duplicate user IDs. Line 637 creates a Set of user IDs, but line 641 subtracts users.length (which can include duplicates) from the member count. This should subtract userIdsToRemove.size instead to accurately reflect the number of unique users being removed.

The usersCount field may be undefined for rooms that were created before this field was added, or if the projection doesn't include it. Line 553 passes room.usersCount directly without a fallback, unlike line 599 which uses room.usersCount || 0. When usersCount is undefined, the calculation on line 641 will result in NaN, causing incorrect logic in the ownership handling. Consider adding a default value like room.usersCount ?? 0 to handle undefined cases consistently.