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

// 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