TypeScript declaration merging and module augmentation

Need to add properties to third-party library types — like adding custom fields to Express Request, extending Window, or augmenting a module's type definitions.

Use declaration merging with module augmentation. For Express: declare module 'express-serve-static-core' { interface Request { userId?: string; } }. For global types: declare global { interface Window { myApp: AppConfig; } }. Put declarations in a .d.ts file included in tsconfig. Important: (1) Must use the exact module name the library uses internally. (2) The file must have at least one import/export to be treated as a module. (3) Use interface (not type) — interfaces merge, types don't.

TypeScript interfaces with the same name in the same scope merge automatically. Module augmentation leverages this to add properties to existing interfaces defined in other modules.