What is covariance and contravariance in TypeScript?

Variance describes how subtyping relationships between complex types relate to subtyping of their component types. In TypeScript: Covariance means "subtype in, subtype out" — if Cat is a subtype of Animal, then Array<Cat> is a subtype of Array<Animal> (you can use a Cat array where an Animal array is expected). Most positions in TypeScript are covariant. Contravariance means the direction is reversed — if Cat extends Animal, a function (a: Animal) => void is a subtype of (c: Cat) => void. This is because a function that can handle any Animal can certainly handle a Cat, but not vice versa. TypeScript enforces contravariance for function parameters with "strictFunctionTypes": true. This matters for callbacks and event handlers — a more general callback is safely substitutable for a more specific one. Methods (unlike function properties) are bivariant in TypeScript for historical compatibility reasons.