Module Augmentation

Module augmentation adds new types to an existing module you've imported — common when a library expects you to extend one of its own interfaces.

// express.d.ts
import "express";

declare module "express" {
    interface Request {
        userId?: string;
    }
}

This adds a userId property to Express's own Request type project-wide, so req.userId type-checks anywhere Request is used — a common pattern for adding authentication middleware's data to a framework's built-in types.