website/content/wiki/typescript/type-challenges.md

title
Type Challenges

4・Pick

Implement the built-in Pick<T, K> generic without using it. Constructs a type by picking the set of properties K from T.

type MyPick<T, K extends keyof T> = {
  [key in K]: k extends keyof T ? T[key] : never;
}

7・Readonly

Implement the built-in Readonly<T> generic without using it. Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned.

type MyReadonly<T> = {
  readonly [key in keyof T]: T[key];
}

11・Tuple to Object

Given an array, transform it into an object type and the key/value must be in the provided array.

type TupleToObject<T extends readonly (string | number)[]> = {
  [val in T[number]]: val;
}