Defined yes, but still arbitrary and may not be consistent between JSON values of the same schema, as per the document you linked to:

> in ascending chronological order of property creation

Also, while JSON came from JS sort-of, it is, for better or worse (better than XML!) a standard apart from JS with its own definitions and used in many other contexts. JSON as specified does not have a prescribed order for properties, so it is not safe to assume one. JS may generally impose a particular order, but other things may not when [re]creating a JSON string from their internal format (JSONB in this case), so by assuming a particular order will be preserved you would be relying on a behaviour that is undefined in the context of JSON.