Challenge 1: Add .post() With a Typed Body — Possible Solution
====================================================================

type ExtractParams<Path extends string> =
  Path extends `${string}:${infer Param}/${infer Rest}`
    ? { [K in Param | keyof ExtractParams<Rest>]: string }
    : Path extends `${string}:${infer Param}`
      ? { [K in Param]: string }
      : {};

type Handler<Path extends string> = (req: { params: ExtractParams<Path> }) => void;
type PostHandler<Path extends string, Body> = (req: { params: ExtractParams<Path>; body: Body }) => void;

type Routes = Record<string, (...args: any[]) => void>;

class Router<R extends Routes = {}> {
  private routes: R;
  constructor(routes: R) {
    this.routes = routes;
  }

  get<Path extends string>(
    path: Path,
    handler: Handler<Path>
  ): Router<R & Record<Path, Handler<Path>>> {
    return new Router({ ...this.routes, [path]: handler } as R & Record<Path, Handler<Path>>);
  }

  post<Path extends string, Body>(
    path: Path,
    handler: PostHandler<Path, Body>
  ): Router<R & Record<Path, PostHandler<Path, Body>>> {
    return new Router({ ...this.routes, [path]: handler } as R & Record<Path, PostHandler<Path, Body>>);
  }
}

// --- Usage ---

interface CreateUserBody {
  name: string;
  email: string;
}

const router = new Router({})
  .get("/users/:id", (req) => {
    console.log(`Fetching user ${req.params.id}`); // params.id: string
  })
  .post<"/users", CreateUserBody>("/users", (req) => {
    console.log(`Creating ${req.body.name} <${req.body.email}>`); // body typed as CreateUserBody
    // req.params is {} here since "/users" has no :params
  });

WHY THIS WORKS
--------------
- PostHandler<Path, Body> adds a second generic dimension (Body) alongside
  the existing Path-derived ExtractParams<Path> — the handler's req object
  now carries both a path-derived shape and an independently-specified one.
- Body isn't inferable from Path the way params are (there's nothing in
  the path string that describes a request body), so it's supplied
  explicitly as a type argument at the call site
  (`.post<"/users", CreateUserBody>(...)`) — a common, honest trade-off
  when a value genuinely can't be derived from something else already
  in scope.
- The Router class itself needed no structural changes to accommodate a
  second route-registration method — `.post()` follows the exact same
  accumulator shape as `.get()` (merge one more entry into R via
  intersection), demonstrating that the pattern from the chapter
  generalizes to more than one kind of route.