TypeScript - Ambient & Module Augmentation

Ambient globals

// globals.d.ts
declare global {
  interface Window { __APP_VERSION__: string }
}
export {};

Module augmentation

// augment-express.d.ts
import 'express';
declare module 'express' {
  interface Request { userId?: string }
}

Ensure your declaration files are included by tsconfig include or referenced via types.