JavaScript fields were the first JavaScripting implemented in Memento -- before triggers, actions, etc. They were introduced to supply similar function as Calculation fields and are designed to be as simple and easy to learn as possible. They are limited to the current library, unless a Link field takes then elsewhere, and to the current entry, so lib() and entry() are assumed, so to speak. No need to refer to them.
JavaScript fields do not have access to the Memento JavaScript Library.