如何编写 PickByValue 类型？

Question

该Pick类型包含在 TypeScript 中。它的实现如下：

type Pick<T, K extends keyof T> = {
  [P in K]: T[P];
};

你将如何编写一个PickByValue类型，以便以下工作：

type Test = {
  includeMe: 'a' as 'a',
  andMe: 'a' as 'a',
  butNotMe: 'b' as 'b',
  orMe: 'b' as 'b'
};

type IncludedKeys = keyof PickByValue<Test, 'a'>;
// IncludedKeys = 'includeMe' | 'andMe'

Answer 1

假设你打算Test是这样的：

type Test = {
  includeMe: 'a',
  andMe: 'a',
  butNotMe: 'b',
  orMe: 'b'
};

并假设你想PickByValue<T, V>给所有的属性亚型的V（所以PickByValue<T, unknown>应该是T），那么你可以定义PickByValue是这样的：

type PickByValue<T, V> = Pick<T, { [K in keyof T]: T[K] extends V ? K : never }[keyof T]>
type TestA = PickByValue<Test, 'a'>; // {includeMe: "a"; andMe: "a"}
type IncludedKeys = keyof PickByValue<Test, 'a'>; // "includeMe" | "andMe"

但是，如果您想要的只是IncludedKeys，那么您可以更直接地使用KeysMatching<T, V>：

type KeysMatching<T, V> = {[K in keyof T]: T[K] extends V ? K : never}[keyof T];
type IncludedKeysDirect = KeysMatching<Test, 'a'> // "includeMe" | "andMe"

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