Subclassing Native Array with Custom Symbol.isConcatSpreadable for Next.js Dynamic Route Graph Flattening

export class RouteCollection extends Array {
  constructor(...routes) {
    super(...routes);
    this[Symbol.isConcatSpreadable] = true;
  }
 
  filterActive() {
    return this.filter(route => !route.disabled);
  }
}
 
export function buildSitemap() {
  const publicRoutes = new RouteCollection({ path: '/' }, { path: '/about' });
  const internalRoutes = [{ path: '/dashboard', disabled: true }, { path: '/settings' }];
  
  const allRoutes = [].concat(publicRoutes, internalRoutes);
  return allRoutes;
}