8-readonly-2.ts

/**
 * 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<T>`.
 *
 * For example
 *
 * ```ts
 * interface Todo {
 *   title: string
 *   description: string
 *   completed: boolean
 * }
 *
 * const todo: MyReadonly2<Todo, 'title' | 'description'> = {
 *   title: "Hey",
 *   description: "foobar",
 *   completed: false,
 * }
 *
 * todo.title = "Hello" // Error: cannot reassign a readonly property
 * todo.description = "barFoo" // Error: cannot reassign a readonly property
 * todo.completed = true // OK
 * ```
 */

/* _____________ Your Code Here _____________ */

type MyReadonly2<T extends object, K extends keyof T = keyof T> = {
  readonly [Key in K]: T[Key];
} & {
  [Key in keyof T as Key extends K ? never : Key]: T[Key];
}

/* _____________ Test Cases _____________ */
import type { Alike, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Alike<MyReadonly2<Todo1>, Readonly<Todo1>>>,
  Expect<Alike<MyReadonly2<Todo1, 'title' | 'description'>, Expected>>,
  Expect<Alike<MyReadonly2<Todo2, 'title' | 'description'>, Expected>>,
  Expect<Alike<MyReadonly2<Todo2, 'description' >, Expected>>,
]

// @ts-expect-error
type error = MyReadonly2<Todo1, 'title' | 'invalid'>

interface Todo1 {
  title: string
  description?: string
  completed: boolean
}

interface Todo2 {
  readonly title: string
  description?: string
  completed: boolean
}

interface Expected {
  readonly title: string
  readonly description?: string
  completed: boolean
}