> 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.

# Visitor

## Overview

Add operations to object structures without changing their classes.

## When to use

- You need to run multiple operations on a stable object structure.
- Adding new operations is more common than adding new node types.

## Java example

```java
interface Visitor {
    void visit(TextNode node);
    void visit(ImageNode node);
}

interface Node {
    void accept(Visitor v);
}

class TextNode implements Node {
    public void accept(Visitor v) { v.visit(this); }
}
```

## TypeScript example

```ts
interface Visitor {
  visitText(node: TextNode): void;
  visitImage(node: ImageNode): void;
}

interface Node {
  accept(v: Visitor): void;
}

class TextNode implements Node {
  accept(v: Visitor): void { v.visitText(this); }
}
```

## Pros and cons

Pros:

- New operations can be added without modifying nodes.
- Separates algorithms from object structure.

Cons:

- Adding new node types requires updating all visitors.
- Can be verbose.

## Common pitfalls

- Using Visitor for small object graphs.
- Forgetting to update all visitors after new nodes.

## Related patterns

- [Iterator](https://nhanbe.id.vn/design-patterns/behavioral/iterator.md)
- [Composite](https://nhanbe.id.vn/design-patterns/structural/composite.md)