Generic type variance in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is covariance in generic types?

Covariance means a generic type preserves the direction of subtyping. If A is a subtype of B, then Generic<A> is a subtype of Generic<B>. It allows safe substitution when reading values.

beginner

What does contravariance mean in generic types?

Contravariance means a generic type reverses the direction of subtyping. If A is a subtype of B, then Generic<B> is a subtype of Generic<A>. It is useful when a generic type is used for input (like function parameters).

beginner

What is invariance in generic types?

Invariance means no subtyping relationship is preserved between generic types. If A is a subtype of B, Generic<A> and Generic<B> are unrelated types. This happens when a generic type is used for both input and output.

intermediate

How does TypeScript handle variance in function types?

TypeScript treats function parameter types as contravariant and return types as covariant. This means you can pass a function expecting a more general parameter where a more specific one is expected, and return a more specific type than expected.

beginner

Why is understanding generic type variance important?

It helps you write safer and more flexible code by knowing when you can substitute one generic type for another. It prevents bugs related to incorrect assumptions about type relationships in generics.

If Dog is a subtype of Animal, which is true for a covariant generic Box<T>?

A <code>Box&lt;Dog&gt;</code> is a subtype of <code>Box&lt;Animal&gt;</code>

B <code>Box&lt;Animal&gt;</code> is a subtype of <code>Box&lt;Dog&gt;</code>

C <code>Box&lt;Dog&gt;</code> and <code>Box&lt;Animal&gt;</code> are unrelated

D Both are the same type

Covariance preserves subtyping direction, so Box<Dog> is a subtype of Box<Animal>.

Which variance applies when a generic type is used only as a function parameter?

A Covariance

B Invariance

C Bivariance

D Contravariance

Function parameters are contravariant because the subtyping direction reverses for inputs.

What happens to variance if a generic type is used both as input and output?

A Contravariant

B Invariant

C Covariant

D Bivariant

Using a generic type both as input and output makes it invariant, so no subtyping relationship is preserved.

In TypeScript, function return types are:

A Covariant

B Contravariant

C Invariant

D Unrelated

Function return types are covariant, meaning they preserve subtyping direction.

Why might ignoring variance cause bugs in TypeScript generics?

A Because it causes syntax errors

B Because it makes code run slower

C Because it can allow unsafe substitutions of types

D Because it prevents using generics

Ignoring variance can allow substituting incompatible types, leading to runtime errors.

Explain covariance, contravariance, and invariance in generic types with simple examples.

Think about how subtyping changes when used for input or output.

Describe how TypeScript treats variance in function types and why it matters.

Consider how functions accept inputs and produce outputs.