Readonly 2

In this medium-level challenge, you'll implement a generic MyReadonly2<T, K> which takes two type arguments:

• T: a type to modify
• K: the keys that should be set to readonly

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

// Video Solution: https://youtube.com/watch?v=kSnFXYvuJV8

/* _____________ Your Code Here _____________ */

type MyReadonly2<T, K> = any

/* _____________ Test Cases _____________ */
import type { Alike, Expect } from '../helpers'

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 Todo

Detailed Explanation

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

How it works:

• First we constrain K to be a subset of the keys of T (keyof T)
• Then we set the default value of K to keyof T, handling the case where K is not provided and is supposed to be all keys of T
• We then create a new type that is a union of the keys in K and the keys in T that are not in K
• The readonly modifier is applied to the keys in K
• The & operator is used to combine the two types

This challenge helps you understand how to use readonly modifiers and how to apply this concept in real-world scenarios.