route-config.ts

type RouteConfig<C extends RouteChildren = {}> = {
  [P in keyof C]: RouteConfig;
} & {
  (): string;
  children: RouteChildren;
};

type RouteChildren = {
  [key: string]: RouteConfig;
};

export function route<C extends RouteChildren>(path: string, children: RouteChildren = {}) {
  const fn: RouteConfig = function() {
    return path
  }
  fn.toString = () => path;
  fn.children = children;
  Object.entries(children).forEach(([key, nested]) => {
    // @ts-ignore
    fn[key] = route(path + nested(), nested.children);
  })
  return fn
}

const routes = {
  root: route('/'),
  home: route('/home', {
    user: route('/user', {
    notifications: route('/notifications'),
    settings: route('/settings'),
  }),
  dashboard: route('/dashboard'),
}),
}

console.log(routes.home.user.settings())