Readonly2

introduction#

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

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

View on GitHub

start point#

ts
/* _____________ Your Code Here _____________ */
 
type MyReadonly2<T, K> = any
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Alike<MyReadonly2<Todo1, 'title' | 'description'>, Expected>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Alike<MyReadonly2<Todo2, 'title' | 'description'>, Expected>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.]
 
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
}
 Try

ts
/* _____________ Your Code Here _____________ */
 
type MyReadonly2<T, K> = any
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Alike<MyReadonly2<Todo1, 'title' | 'description'>, Expected>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Alike<MyReadonly2<Todo2, 'title' | 'description'>, Expected>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.]
 
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
}
 Try

take the challenge

my solutions#

Spoiler warning // Click to reveal answer

ts
type MyOmit<T, K extends string | number | symbol> = {
  [Key in Exclude<keyof T, K>]: T[Key]
}
 
type MyReadonly2<T, K extends keyof T = keyof T> = {
  readonly [Key in K]: T[Key]
} & MyOmit<T, K>
 Try

ts
type MyOmit<T, K extends string | number | symbol> = {
  [Key in Exclude<keyof T, K>]: T[Key]
}
 
type MyReadonly2<T, K extends keyof T = keyof T> = {
  readonly [Key in K]: T[Key]
} & MyOmit<T, K>
 Try

view more solutions