What does the following generic constraint pattern achieve? type Covariant = () => T; type Contravariant = (arg: T) => void; // Which is wider: Covariant or Covariant if Dog extends Animal?

Question

TypeScript Professional — Hard

What does the following generic constraint pattern achieve? type Covariant = () => T; type Contravariant = (arg: T) => void; // Which is wider: Covariant or Covariant if Dog extends Animal?

Accepted Answer

The generic constraint pattern in the question allows the Covariant type to be wider than Contravariant. This is because returning a subtype (Dog) in Covariant satisfies the expectation of returning a supertype (Animal).