> 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.

# Decorator

## Overview

Attach additional responsibilities to an object dynamically by wrapping it.

## When to use

- You need flexible, runtime composition of behavior.
- Inheritance would lead to too many subclasses.

## Java example

```java
interface Notifier {
    void send(String message);
}

class EmailNotifier implements Notifier {
    public void send(String message) { /* email */ }
}

class SmsDecorator implements Notifier {
    private final Notifier wrapped;
    SmsDecorator(Notifier wrapped) { this.wrapped = wrapped; }
    public void send(String message) {
        wrapped.send(message);
        /* sms */
    }
}
```

## TypeScript example

```ts
interface Notifier {
  send(message: string): void;
}

class EmailNotifier implements Notifier {
  send(message: string): void {}
}

class SmsDecorator implements Notifier {
  constructor(private wrapped: Notifier) {}
  send(message: string): void {
    this.wrapped.send(message);
  }
}
```

## Pros and cons

Pros:

- Behavior can be added without subclassing.
- Multiple decorators can be combined.

Cons:

- Many small objects can be hard to debug.
- Order of decorators matters.

## Common pitfalls

- Hiding side effects in deep decorator stacks.
- Mutating shared state across decorators.

## Related patterns

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