Keyof operator in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the keyof operator do in TypeScript?

The keyof operator creates a union type of all the keys (property names) of a given object type.

beginner

Given type Person = { name: string; age: number; }, what is keyof Person?

keyof Person is the union type 'name' | 'age', representing the keys of the Person type.

intermediate

How can keyof help in writing safer functions?

Using keyof lets you restrict function parameters to only valid keys of an object, preventing errors from invalid property names.

advanced

What is the result of keyof any in TypeScript?

keyof any is the union of string | number | symbol, because any object key can be one of these types.

beginner

Can keyof be used with interfaces as well as types?

Yes, keyof works with both interfaces and type aliases to get the keys of the defined shape.

What type does keyof { a: number; b: string; } produce?

A object

B number | string

C 'a' | 'b'

D any

keyof produces a union of the keys, so here it is 'a' | 'b'.

Which of these is a valid use of keyof?

A To get the values of an object

B To convert a type to a string

C To create a new object

D To get the keys of a type as a union

keyof extracts keys from a type as a union of string literal types.

If type T = { x: number; y: number; }, what is keyof T?

A 'x' | 'y'

B number

C object

D string

keyof T is the union of the keys: 'x' | 'y'.

What does keyof any represent?

A string | number | symbol

B any

C object

D never

keyof any is the union of all possible key types: string | number | symbol.

Can keyof be used on primitive types like string?

A No, it only works on objects

B Yes, it returns keys of the wrapper object

C Yes, but returns never

D No, it causes an error

Primitive types have wrapper objects with keys, so keyof string returns keys like length.

Explain what the keyof operator does and give an example.

Think about how you get property names from a type.

How can using keyof improve type safety in functions?

Consider how you can limit inputs to only existing keys.