Module Augmentation: Adding Types to External Libraries

Module augmentation is like monkey-patching for types. It allows you to safely add your own type definitions to external modules you don't control, making TypeScript aware of your custom extensions. For example, you can augment Express's `Request` type to include a `user` property after authentication middleware runs. The main footgun is getting the syntax wrong: you must wrap your extension in a `declare module 'module-name'` block. Without it, TypeScript thinks you're creating a brand new module, and the augmentation won't apply.