get
Returns the index number for this message.
By design of the conversations system the message indices may have arbitrary gaps between them, that does not necessarily mean they were deleted or otherwise modified - just that messages may have non-contiguous indices even if they are sent immediately one after another.
Trying to use indices for some calculations is going to be unreliable.
To calculate the number of unread messages it is better to use the read horizon API. See getUnreadMessagesCount for details.
Return
the message index.