🎯

mirror-types

🎯Skill

from marius-townhouse/effective-typescript-skills

What it does

Mirrors external types locally to prevent tight coupling and isolate code from external type changes.

📦

Part of

marius-townhouse/effective-typescript-skills(83 items)

mirror-types

Installation

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills --all

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills -s prefer-unknown-over-any exhaustiveness-checking

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills -a opencode claude-code

Quick InstallInstall with npx

npx skills add marius-townhouse/effective-typescript-skills -l

git cloneClone repository

git clone https://github.com/marius-townhouse/effective-typescript-skills.git

📖 Extracted from docs: marius-townhouse/effective-typescript-skills

Need more details? View full documentation on GitHub →

1Installs

AddedFeb 4, 2026

View on GitHub Back to Skills

Skill Details

SKILL.md

Use when depending on external types. Use when avoiding tight coupling. Use when external types might change. Use when building adapters. Use when types are only used internally.

Overview

# Mirror Types to Sever Dependencies

Overview

When you depend on types from external packages, changes to those types can break your code. Mirroring types - creating your own local copies of external types - severs this dependency. This is useful when you only need a subset of external types or when you want to insulate yourself from external changes.

When to Use This Skill

Depending on external types
Avoiding tight coupling to external packages
External types might change frequently
Building adapters or wrappers
Types are only used internally

The Iron Rule

Mirror external types when you want to sever dependencies. Define only the subset you need, insulated from external changes.

Example

```typescript

// BAD: Direct dependency on external type

import { ExternalUser } from 'external-library';

interface MyService {

processUser(user: ExternalUser): void;

}

// If ExternalUser changes, your code breaks

// GOOD: Mirror the type

interface User {

id: string;

name: string;

email: string;

}

interface MyService {

processUser(user: User): void;

}

// Adapter converts external to internal type

function adaptExternalUser(external: ExternalUser): User {

return {

id: external.id,

name: external.name,

email: external.email,

};

}

```

Reference

Effective TypeScript, 2nd Edition by Dan Vanderkam
Item 70: Mirror Types to Sever Dependencies

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