```ts
export interface User {
  email: string;
  active: boolean;
}

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