Basic mapped type syntax in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mapped type in TypeScript?

A mapped type creates a new type by transforming each property of an existing type using a template. It lets you apply the same operation to all properties easily.

beginner

Syntax of a basic mapped type

type NewType = { [Key in keyof OldType]: OldType[Key] }<br>This means: for each property Key in OldType, create a property Key with the same type.

beginner

What does keyof do in mapped types?

keyof extracts all property names (keys) of a type as a union of string literal types. It helps to loop over all keys in a mapped type.

intermediate

Example: Make all properties optional using mapped types

type Partial<T> = { [P in keyof T]?: T[P] }<br>This creates a new type where all properties of T are optional.

intermediate

Can mapped types change property types?

Yes! You can transform property types. For example, { [P in keyof T]: string } makes all properties have type string regardless of original type.

What does keyof return when used on a type?

A The type's methods only

B The type's values

C The type's constructor

D A union of the type's property names

keyof returns a union of all property names (keys) of a type.

Which syntax correctly defines a mapped type over type T?

A { [P in keyof T]: T[P] }

B { T in keyof P: P[T] }

C { [T of keyof P]: P[T] }

D { [P of T]: keyof P }

The correct syntax uses [P in keyof T] to iterate keys.

How to make all properties of a type optional using mapped types?

A { optional [P in keyof T]: T[P] }

B { [P in keyof T]?: T[P] }

C { [P in keyof T]: optional T[P] }

D { [P in keyof T]: T[P]? }

Adding ? after the property name makes it optional.

Can mapped types change the type of properties?

A Yes, they can assign new types to properties

B No, they only copy original types

C Only for string properties

D Only for number properties

Mapped types can transform property types to any new type.

What is the purpose of mapped types?

A To define classes

B To execute functions on objects

C To create new types by transforming properties of existing types

D To import modules

Mapped types help create new types by applying changes to each property of an existing type.

Explain how a basic mapped type works in TypeScript and give a simple example.

Think about looping over keys and creating a new type with the same properties.

Describe how you can use mapped types to make all properties of a type optional.

Focus on the question mark after the property name in the mapped type.