Export All Types That Appear in Public APIs

Overview

When you publish a library, users need access to all types that appear in your public API. If a function returns User but User isn’t exported, users can’t type their own variables to match. Export all types that appear in public function signatures, return types, or interfaces.

This is essential for good developer experience in libraries.

When to Use This Skill

• Publishing TypeScript libraries
• Types appear in public function signatures
• Users need to reference your types
• Building reusable components
• Designing library interfaces

The Iron Rule

Export every type that appears in your public API. Users need these types to work with your library effectively.

Example

// BAD: Internal type not exported
interface User {
  id: string;
  name: string;
}

export function getUser(id: string): User {
  // ...
}

// User can't do:
// import { getUser, User } from 'library'; // Error: User not exported
// const user: User = getUser('123');

// GOOD: Export the type
export interface User {
  id: string;
  name: string;
}

export function getUser(id: string): User {
  // ...
}

// User can now:
// import { getUser, User } from 'library';
// const user: User = getUser('123');

Reference

• Effective TypeScript, 2nd Edition by Dan Vanderkam
• Item 67: Export All Types That Appear in Public APIs