```ts
import { strict as assert } from 'assert';

export interface User {
  email: string;
  active?: boolean; // Optional property to be considered "active" if present
}

/** 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());
}
```