> For AI agents: the complete documentation index is available at https://nhanbe.id.vn/llms.txt, the full documentation bundle is available at https://nhanbe.id.vn/llms-full.txt.

# Memento

## Overview

Capture and externalize an object's internal state so it can be restored later.

## When to use

- You need undo/redo.
- You want to keep state history without exposing internals.

## Java example

```java
class EditorMemento {
    private final String content;
    EditorMemento(String content) { this.content = content; }
    String getContent() { return content; }
}

class Editor {
    private String content = "";
    void type(String text) { content += text; }
    EditorMemento save() { return new EditorMemento(content); }
    void restore(EditorMemento memento) { content = memento.getContent(); }
}
```

## TypeScript example

```ts
class EditorMemento {
  constructor(readonly content: string) {}
}

class Editor {
  private content = "";
  type(text: string): void { this.content += text; }
  save(): EditorMemento { return new EditorMemento(this.content); }
  restore(m: EditorMemento): void { this.content = m.content; }
}
```

## Pros and cons

Pros:

- Enables undo/redo without exposing internals.
- Clear separation of state snapshots.

Cons:

- Memory usage can grow quickly.
- Snapshot creation may be expensive.

## Common pitfalls

- Saving too frequently without limits.
- Storing large, deep object graphs.

## Related patterns

- [Command](https://nhanbe.id.vn/design-patterns/behavioral/command.md)
- [State](https://nhanbe.id.vn/design-patterns/behavioral/state.md)