Recap: Memento Pattern

Memento saves an object's state for later without making that state public.

• Intent. Capture an object's internal state so it can be restored, without breaking encapsulation.
• Participants. The originator makes and reads mementos; the memento holds the state; the caretaker keeps mementos and never reads them.
• Sealing it. A nested class with private fields in Java, a #private field in TypeScript, a leading underscore by convention in Python.
• Copy, do not share. A memento must copy mutable state, or it changes with the originator.
• Against Command. A command undoes by reversing its change; a memento undoes by copying the old state back.
• The trap. Full snapshots on every change fill the memory; cap the history or store only the changes.
• When not. Large state that changes constantly, or state that cannot be copied back.