Mapped Typeの修飾子：readonlyと?

ようこそ

+による修飾子の追加

type Readonly<T> = { +readonly [K in keyof T]: T[K] }; // + is implicit
type Optional<T> = { [K in keyof T]+?: T[K] };

-によるreadonlyの削除

type Mutable<T> = { -readonly [K in keyof T]: T[K] };
const mutableUser: Mutable<Readonly<User>> = { id: 1, name: 'Alice' };
mutableUser.id = 2; // OK now

-による?の削除

type Required<T> = { [K in keyof T]-?: T[K] };
// Now all optional properties become required

-と-?の組み合わせ

type Concrete<T> = { -readonly [K in keyof T]-?: T[K] };

PartialとRequiredは正反対

// From TypeScript lib:
type Partial<T> = { [K in keyof T]?: T[K] };   // adds ?
type Required<T> = { [K in keyof T]-?: T[K] };  // removes ?

デフォルトでの修飾子の維持

interface User { readonly id: number; name?: string; }
type Copy<T> = { [K in keyof T]: T[K] };
type C = Copy<User>; // readonly id preserved, name? preserved

実践：Patch型

type Patch<T> = { -readonly [K in keyof T]?: T[K] };

DeepReadonly

type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K]
};

DeepPartial

type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K]
};

修飾子の組み合わせ表

確認問題

まとめ