[3주차] Medium 2, 3, 8, 9, 10

[Choozii]

https://github.com/type-challenges/type-challenges/blob/main/README.md

2. Get Return Type ✅

Implement the built-in ReturnType generic without using it.

type MyReturnType<T extends (arg: any, ...args: any) => any> = T extends (arg: any, ...args: any) => infer R ? R : unknown

3. Omit 👀

Implement the built-in Omit<T, K> generic without using it. Constructs a type by picking all properties from T and then removing K

type MyOmit<T, K extends keyof T> = {[P in keyof T extends K? never: P]: T[P]}
// 찾아본 정답
type MyOmit<T, K extends keyof T> = {[P in keyof T as P extends K? never: P]: T[P]}
// tslib
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

• as에 대해 공부 필요...

8. Readonly 2 👀

Implement a generic MyReadonly2<T, K> which takes two type argument T and K. K specify the set of properties of T that should set to Readonly. When K is not provided, it should make all properties readonly just like the normal Readonly.

// 내 답 -> error 있음 (omit을 이용해봤으나...)
type MyReadonly2<T, K extends keyof T> = { readonly [P in K]: T[P] } & { [P in keyof T as P extends K ? never: P]: T[P]}
// 답이 되려면
type MyReadonly2<T, K extends keyof T = keyof T> = { readonly [P in K]: T[P] } & { [P in keyof T as P extends K ? never: P]: T[P]}
// = keyof T가 추가되어야 한다

9. Deep Readonly👀

Implement a generic DeepReadonly which make every parameter of an object - and its sub-objects recursively - readonly.

type DeepReadonly<T> = {
  readonly [P in keyof T]: keyof T[P] extends never ? T[P] : DeepReadonly<T[P]>
}

10. Tuple to Union ✅

type TupleToUnion<T extends any[]> = T[number]