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

/** The lowercased addresses of the active users, in order. */
export function activeEmails(users: readonly User[]): string[] {
  /**
   * Returns the lowercased addresses of the active users, in input order.
   *
   * @param {readonly User[]} users
   * @returns {string[]}
   */
  return users.map(user => user.active ? user.email.toLowerCase() : '');
}

```