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

export interface User {
  email: string;
  active?: boolean; // Optional by default to accommodate no `active` property
}

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