Summary - Mapped types
- TypeScript course
- Intermediate
- Lesson 35/89
- 4 min
Write your own utility type with mapped types.
Summary - Mapped types
Mapped types iterate over a type's properties and transform each one. With [K in keyof T]: T[K] you copy properties; readonly makes them immutable; ? makes them optional. Combining the three you can write your own utility types.
type Guvenli<T> = {
readonly [K in keyof T]?: T[K];
};Practice Your own utility type
Write the mapped type Guvenli<T> as { readonly [K in keyof T]?: T[K] }. Define interface Ayar with tema: string and ses: number. Define type GuvenliAyar as Guvenli<Ayar>. Assign { tema: "koyu" } to a variable a (of type GuvenliAyar). Print a.tema with console.log.
Starter code
// Guvenli<T> mapped type tanımla
// T'nin tüm özelliklerini hem readonly hem isteğe bağlı yap
// Ayar arayüzünü tema ve ses ile tanımla
// GuvenliAyar türünü Guvenli<Ayar> olarak tanımla
// a değişkenine yalnızca tema içeren nesneyi ata
// tema değerini yazdırSample solution
type Guvenli<T> = {
readonly [K in keyof T]?: T[K];
};
interface Ayar {
tema: string;
ses: number;
}
type GuvenliAyar = Guvenli<Ayar>;
const a: GuvenliAyar = { tema: "koyu" };
console.log(a.tema);Expected output
koyu
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 You want both immutability and optionality in a mapped type. Which form is correct?
- { [K in keyof T]: T[K]? readonly }
- { [readonly K in keyof T]?: T[K] }
- { [K in keyof T]: readonly T[K] }
- { readonly [K in keyof T]?: T[K] } Correct answer
Explanation: readonly goes before the square brackets and ? goes right after. This way each property is both immutable and optional. The other forms are syntax errors.
2 Combined mapped type body
type Guvenli<T> = {
____ [K in keyof T]?: T[K];
};type Guvenli<T> = {
readonly [K in keyof T]?: T[K];
};Explanation: To make all properties immutable you write readonly before the square brackets. ? already handles optionality; optional is not a keyword; static and const serve other purposes.
3 Match mapped type transforms
| Term | Meaning |
|---|---|
[K in keyof T]: T[K] |
Copies all properties as-is |
readonly [K in keyof T]: T[K] |
Makes all properties immutable |
[K in keyof T]?: T[K] |
Makes all properties optional |
readonly [K in keyof T]?: T[K] |
Both immutable and optional |