Función Pura.

Definition

A function that always returns the same output for the same inputs and produces no observable effect beyond that — it doesn't write to a database, read a clock, or mutate external state. Because it's deterministic, it can be tested with plain asserts, no mocks or infrastructure needed. Keeping the core logic pure and pushing side effects to a thin outer layer is what makes a "pure engine" architecture cheap to test and safe to refactor.