# Medium - Readonly key

# question

Implement a generic MyReadonlyKey<T, K> which takes two type argument T and K.

实现一个通用的 MyReadonlyKey<T, K>，它带有两种类型的参数 T 和 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>.

K 指定应设置为 Readonly 的 T 的属性集。如果未提供 K，则应使所有属性都变为只读，就像普通的 Readonly<T> 一样。

For example

interface Todo {
  title: string
  description: string
  completed: boolean
}

const todo: MyReadonlyKey<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

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# answer

实现一

type MyReadonlyKey<T, K extends keyof T = keyof T> = T & { readonly [P in K]: T[P] }

实现二

type Diff<A, B> = A extends B ? never : A

type MyReadonlyKey<T, K extends keyof T = keyof T> =
  { readonly [S in K]: T[S] }
  & { [S in Diff<keyof T, K>]: T[S] }

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# tips

和函数一样，参数可以提供默认值 keyof T

← 实现 Pick 实现 Readonly →