Advanced TypeScript

Mapped Types

Mapped types transform object types by iterating over keys. They are how TypeScript expresses mechanical object-shape changes such as readonly, optional, required, picked, omitted, and remapped properties.

Mapped types iterate over keys
Indexed access gets property value types
Modifiers can be added or removed
Keys can be remapped
Mechanical transformation is not semantic design

Basic mapped types

type Mutable<T> = {
  -readonly [K in keyof T]: T[K]
}

type Nullable<T> = {
  [K in keyof T]: T[K] | null
}

The new type follows the keys of the original type.

Key remapping

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K]
}

type UserGetters = Getters<{ id: string; name: string }>
// { getId: () => string; getName: () => string }

Mapped types can generate new property names from old ones.

Mapped type tools
Tool	Use
keyof T	Get key union
K in keyof T	Iterate keys
T[K]	Read property value type
readonly / -readonly	Add/remove readonly
? / -?	Add/remove optional
as	Remap/filter keys