typescript - 根据值类型过滤键子列表的接口(interface)键

Question

问题

给定一个被视为 map 的 TypeScript 接口(interface)(将唯一键类型与非唯一值类型相关联)，可以提取映射到指定值类型的键类型？

作为一个具体的例子，从 lib.dom.d.ts 中的 WindowEventMap 开始......

interface WindowEventMap 
    extends GlobalEventHandlersEventMap, WindowEventHandlersEventMap {
    "abort": UIEvent;
    "afterprint": Event;
    "beforeprint": Event;
    "beforeunload": BeforeUnloadEvent;
    "blur": FocusEvent;
    // ...
}

有一个内置的运算符keyof 用于获取所有的 key :

type FullKeyList = keyof WindowEventMap;
// gives: FullKeyList = "'abort' | 'afterprint' | 'beforeprint' | ..and 93 more"

是否有一些表达式可能只过滤映射到值类型 KeyboardEvent 的键？:

// Goal:
type SubKeyList = SomeExpression<WindowEventMap, KeyboardEvent>;
// would give: SubKeyList = "'keyup' | 'keypress' | 'keydown'"

解决方案尝试

TypeScript 具有条件类型表达式:

T extends U ? X : Y

所以这是一个合理的尝试:

type Filter<K extends keyof M, V, M> = M[K] extends V ? K : never;

它过滤个别类型:

// Passes this test:
type FilteredExample1 = Filter<'keyup', KeyboardEvent, WindowEventMap>;
// gives FilteredExample1 = "'keyup'"

// Passes this test:
type FilteredExample2 = Filter<'blur', KeyboardEvent, WindowEventMap>;
// gives FilteredExample2 = "never"

它可以过滤类型的联合并返回一个新的联合吗？开始看起来不错:

// Passes this test:
type FilteredUnionExample1 = Filter<'keyup' | 'keydown', KeyboardEvent , WindowEventMap>;
// gives FilteredUnionExample1 = "'keyup' | 'keydown'"

但是，如果一个或多个联合体成员失败，则联合体失败:

// Fails this test:
type FilteredUnionExample2 = Filter<'keyup' | 'keydown' | 'blur', KeyboardEvent, WindowEventMap>;
// gives FilteredUnionExample2 = "never" (and not sub-union "'keyup' | 'keydown'")

// And so, it also fails the end-goal usage:
type AllKeysUnion = keyof WindowEventMap;
type FilteredUnionExample3 = Filter<AllKeysUnion, KeyboardEvent , WindowEventMap>;
// gives FilteredUnionExample3 = "never" (not sub-union "'keyup' | 'keypress' | 'keydown'")

这个问题有解决办法吗？

Answer 1

我通常称之为KeysMatching:

type KeysMatching<T, V> = {[K in keyof T]: T[K] extends V ? K : never}[keyof T];

type SubKeyList = KeysMatching<WindowEventMap, KeyboardEvent>
// type SubkeyList = "keydown" | "keypress" | "keyup"

希望对您有所帮助；祝你好运!

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