Covariance and contravariance in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is covariance in TypeScript?

Covariance means you can use a more specific type than expected. For example, if a function expects a type, you can give it a subtype. It works like a "child" fitting where a "parent" is expected.

beginner

What is contravariance in TypeScript?

Contravariance means you can use a more general type than expected. For example, if a function expects a subtype, you can give it a supertype. It is like accepting a broader category where a specific one is expected.

intermediate

Explain covariance with function return types.

Function return types are covariant. This means a function that returns a subtype can be used where a function returning a supertype is expected. It is safe because the caller expects a general type but gets a more specific one.

intermediate

Explain contravariance with function parameter types.

Function parameter types are contravariant. This means a function that accepts a supertype can be used where a function expecting a subtype is required. It is safe because the function can handle more general inputs.

beginner

Why is understanding covariance and contravariance important in TypeScript?

It helps you write flexible and safe code. You can substitute types correctly in functions and variables without errors. It also helps understand how TypeScript checks type compatibility.

Which of the following is an example of covariance in TypeScript?

A A function returning a supertype where a subtype is expected

B A function accepting a supertype where a subtype is expected

C A function accepting a subtype where a supertype is expected

D A function returning a subtype where a supertype is expected

Covariance applies to return types, allowing a function to return a more specific type than expected.

Contravariance in TypeScript applies mainly to:

A Function parameter types

B Class inheritance

C Variable declarations

D Function return types

Contravariance applies to function parameter types, allowing broader types to be accepted.

If a function expects a parameter of type 'Dog', which parameter type can be used contravariantly?

A Animal (supertype of Dog)

B Dog

C Cat (unrelated type)

D string

Contravariance allows using a supertype (Animal) where a subtype (Dog) is expected.

Why is covariance safe for function return types?

A Because the caller expects a more specific type

B Because the caller expects a general type and gets a more specific one

C Because the function accepts more general inputs

D Because the function accepts more specific inputs

The caller expects a general type but receives a more specific one, which is safe.

Which statement about TypeScript variance is true?

A Function parameters are covariant

B Function return types are contravariant

C Function parameters are contravariant

D Variables are always invariant

Function parameters are contravariant, meaning they accept broader types.

Describe covariance and contravariance in your own words with simple examples.

Think about how types can be substituted in function parameters and return values.

Explain why function return types are covariant and function parameter types are contravariant in TypeScript.

Consider what happens if you give a function a more general or more specific type.