```typescript
export interface User {
  email: string;
  active?: boolean; // Adjust comment based on absence of 'active' property
}

/**
 * The lowercased addresses of the active users, in order.
 */
export function activeEmails(users: readonly User[]): string[] {
  return users.filter((user) => user.active).map((user) => user.email.toLowerCase());
}
```