In TypeScript, generic functions are by default contravariant in the parameter types and covariant in the return type:
Therefore, the following is acceptable:
```typescript
class User {
username: string;
constructor(username: string) {
this.username = username;
}
}
class Admin extends User {
isSuperAdmin: boolean;
constructor(username: string, isSuperAdmin: boolean) {
super(username);
this.isSuperAdmin = isSuperAdmin;
}
}
// Covariance
function logUsername(user: User): void {
console.log(user.username);
}
logUsername(new Admin('admin1', true));
// Contravariance
const admins: Admin[] = [
new Admin('john.smith', false),
new Admin('jane.doe', true),
new Admin('joker', false)
];
const jokers = admins.filter((user: User): boolean => {
return user.username.startsWith('joker');
});
```