Declaration Merging in TypeScript

Interface Merging

// First declaration
interface User {
  name: string;
}

// Second declaration — merged
interface User {
  age: number;
}

// Result:
const user: User = { name: "Alice", age: 30 }; // Both required

Module Augmentation

import 'express';

declare module 'express' {
  interface Request {
    user?: { id: string; role: string };
    requestId: string;
  }
}

// Now Request has user and requestId globally
app.use((req, res, next) => {
  console.log(req.requestId); // OK
  next();
});

Global Augmentation

declare global {
  interface Window {
    __APP_CONFIG__: { apiUrl: string; env: string };
  }
  namespace NodeJS {
    interface ProcessEnv {
      MY_SECRET: string;
    }
  }
}

When to Use Declaration Merging

Use Case	Example
Adding properties to 3rd party types	Express Request.user
Environment variables typing	process.env types
Global window properties	window.APP_CONFIG
Splitting large interfaces	Across multiple files

Interview Questions

Easy: What is declaration merging?

Medium: How do you use module augmentation to extend Express Request?

Hard: What are the risks of declaration merging?