type-challenges (初級)

4. Pick

Utility Typeの1つであるPickを自前で実装する。

https://github.com/type-challenges/type-challenges/blob/main/questions/00004-easy-pick/README.ja.md

解答例

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

type MyPick<T, K extends keyof T = keyof T> = {
  [key in K]: T[key]
}

type TodoPreview = MyPick<Todo, 'title' | 'completed'>

const todo: TodoPreview = {
  title: 'Clean room',
  completed: false
}

7. Readonly

https://github.com/type-challenges/type-challenges/blob/main/questions/00007-easy-readonly/README.ja.md

interface Todo {
  title: string
  description: string
}

type MyReadonly<T> = {
  readonly [key in keyof T]: T[key]
}

const todo: MyReadonly<Todo> = {
  title: 'Hey',
  description: 'foobar'
}

todo.title = 'Hello' // Error: cannot reassign a readonly property
todo.description = 'barFoo' // Error: cannot reassign a readonly property