Memento

A memento works like a save point in a video game. Before a tough boss fight, you save your progress. If things go wrong, you don't restart the whole game. You just load that save point and you're back where you were. In code, a memento is a stored snapshot of an object's data. You can restore it later — that's how undo works.

1. 1The object you want to protect can make a 'memento' — a snapshot copy of its current data.
2. 2You keep that memento somewhere safe: a variable, or a stack of past snapshots for multi-step undo.
3. 3To undo, you hand the memento back to the object, and it copies that old data back over its current data.

Use this when you want undo/redo, checkpoints, or the ability to roll back to an earlier state. Examples: text editors, games, or form drafts.

• ⚠Save a real separate copy, not just a reference to the live object. Otherwise 'the snapshot' keeps changing along with the original.
• ⚠Don't save a snapshot after every tiny change. Save at meaningful checkpoints, or you'll waste memory on saves nobody restores.