```ts
export interface User {
  email: string;
  active?: boolean; // Optional since `email` is required
}

/**
 * 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());
}
```
The new content uses the `.filter()` array method to select only active users and then applies `map()` to convert their emails to lowercase.