Challenge 3: Type systems

Solve a task combining mapped, conditional, and infer.

Type systems challenge

In this challenge you will solve tasks that combine mapped types, conditional types, and the infer keyword. You learned to transform all fields of a type with mapped types, build type-based logic with conditional types, and extract one type from another with infer.

Exercise ReadOnly mapped type

Define a mapped type named ReadOnly: { readonly [K in keyof T]: T[K] }. The Nokta interface should contain x: number and y: number. The p variable should be of type ReadOnly<Nokta> with value { x: 3, y: 4 }. Print p.x + p.y with console.log.

Show example solution

type ReadOnly<T> = { readonly [K in keyof T]: T[K] };
interface Nokta { x: number; y: number; }
const p: ReadOnly<Nokta> = { x: 3, y: 4 };
console.log(p.x + p.y);

Exercise Element type with infer

Define a conditional type named Eleman: T extends (infer U)[] ? U : never. Define R as Eleman<string[]>. The deger variable should be of type R with value "merhaba". Print deger with console.log.

Show example solution

type Eleman<T> = T extends (infer U)[] ? U : never;
type R = Eleman<string[]>;
const deger: R = "merhaba";
console.log(deger);

Test yourself

Which keyword is used to extract the value type from a Promise in a conditional type?
- typeof
- keyof
- infer
- extends
Show answer

infer — The infer keyword performs type inference inside a conditional type; for example T extends Promise<infer U> ? U : never.

Type extraction with infer

type Eleman<T> = T extends (____ U)[] ? U : never;

Show answer

type Eleman<T> = T extends (infer U)[] ? U : never;

Type system concepts

mapped

[K in keyof T]
conditional

T extends U ? X : Y
infer

Type extraction
keyof

Key set

Next lesson: Challenge 4: Mini app

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