P in 型 as ..

Mapped Typesのkeyに対しasを使える

P in KとP in keyof Tは異なる挙動をするという話があるが、asはそれをそのまま引き継ぐ

なので、↑の違いを理解しておけば、as単体で見れば区別して理解する必要はない

P in keyof T as ..の例

code:ts

type Getters<T> = {

[P in keyof T as get${Capitalize<string & P>}]: () => TP;

};

interface Person {

name: string;

age: number;

location: string;

}

type LazyPerson = Getters<Person>;

// type LazyPerson = {

// getName: () => string;

// getAge: () => number;

// getLocation: () => string;

// }

P in K as ..の例

code:ts

type Handlers<Events extends string> = {

[E in Events as on${Capitalize<E>}]?: (event: E) => void;

};

type H = Handlers<'click' | 'hover'>;

// type H = {

// onClick?: (event: "click") => void;

// onHover?: (event: "hover") => void;

// }

一種なので別にリンクにする必要ないかもしれないが一応mrsekut.icon

code:ts

type OnlyIdKeys<K extends PropertyKey> = {

[P in K as P extends ${string}Id ? P : never]: true;

};

type A = OnlyIdKeys<'userId' | 'name' | 'postId'>;

// type A = {

// userId: true;

// postId: true;

// }