Declaration Merging & Module Augmentation

30‑second interview answer

TypeScript can merge interface declarations with the same name—useful for extending existing types. Module augmentation lets you add types to third-party modules (e.g., adding fields to Express.Request). This is a safe way to extend library types without forking them.

Interface merging

interface Config {
  timeout: number;
}
 
interface Config {
  baseUrl: string;
}
 
// Config becomes { timeout: number; baseUrl: string }

Types (type) do not merge.

Module augmentation (common in backend + auth)

Example: extend Express request:

// types/express.d.ts
import 'express-serve-static-core';
 
declare module 'express-serve-static-core' {
  interface Request {
    userId?: string;
  }
}

Now req.userId is typed.

When to use it

• adding custom fields provided by middleware
• augmenting global types (rare, do carefully)

Pitfalls

• augmentations must be included by TS (tsconfig include)
• name collisions if you over-augment

Mini Q&A

Q1: Why can interfaces merge but types can’t?

• Language design: interfaces are open/extendable.

Q2: What is module augmentation?

• Extending types for an imported module.

Q3: Is it runtime behavior?

• No, types only.

Summary checklist

• I can show interface merging.
• I can augment a module safely.
• I understand it’s compile-time only.