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Refer: Medium - Readonly-2

Relation: Easy - Readonly

Describe

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

code snippetCopyjavascript
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

Test Cases

code snippetCopytypescript
import { 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>>,
]

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
}

Solution

code snippetCopytypescript
// my solution
type MyReadonly2<T, K = keyof T> = {
  readonly [P in keyof T as P extends K ? P : never]: T[P]
} & {
  [P in keyof T as P extends K ? never : P]: T[P]
}

// other solution
type MyReadonly2<T, K extends keyof T> = Omit<T, K> & Readonly<Pick<T, K>> // trickšŸ¤”