# Module Augmentation

> Use when extending third-party types. Use when adding properties to existing interfaces. Use when plugins extend core types. Use when declaration merging is needed. Use when augmenting global types.

- Skill: `marius-townhouse/module-augmentation` (Agent Skill)
- Install (CLI): `npx skillmds@latest add marius-townhouse/module-augmentation`
- Raw SKILL.md: https://api.skillmd.com/api/skills/marius-townhouse/module-augmentation/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: marius-townhouse (https://skillmd.com/u/marius-townhouse)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/marius-townhouse/module-augmentation

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# Use Module Augmentation to Improve Types

## Overview

Module augmentation allows you to add declarations to existing modules, including third-party libraries. This is useful when a library's types are incomplete or when you're extending a library with plugins. Use `declare module` to add properties, methods, or types to existing modules.

## When to Use This Skill

- Extending third-party types
- Adding properties to existing interfaces
- Plugins that extend core types
- Declaration merging needed
- Augmenting global types

## The Iron Rule

**Use module augmentation to add to existing types. Declare the same module name and add your declarations.**

## Example

```typescript
// Adding to an existing module
// types/vue.d.ts
declare module 'vue' {
  export interface ComponentCustomProperties {
    $myProperty: string;
    $myMethod(): void;
  }
}

// Now Vue components have these properties
// this.$myProperty works with type safety

// Augmenting global types
declare global {
  interface Window {
    myLib: MyLibrary;
  }
}

// Now window.myLib is typed
```

## Reference

- Effective TypeScript, 2nd Edition by Dan Vanderkam
- Item 71: Use Module Augmentation to Improve Types