Typescript:How 一般获取 class 成员的类型

Question

class Awesome<ObjType extends Object,
     KeyType extends keyof ObjType , 
     MemberType /* is of type obj:ObjType obj[KeyType]*/>{}

这里如何约束MemberType作为ObjType

实例的成员类型

Answer 1

我想你正在寻找 MemberType extends ObjType[KeyType]。

... typing constructs that enable static validation of code involving dynamic property names and properties selected by such dynamic names ...

— from the Pull Request that implemented the feature

对于您的代码，它是这样工作的：

class Awesome<ObjType extends Object,
     KeyType extends keyof ObjType , 
     MemberType extends ObjType[KeyType]> { }

interface A { 
    a: number;
    b: string;
}

// okay:
type A1 = Awesome<A, keyof A, string | number>

// not okay:
type A2 = Awesome<A, keyof A, boolean>

// okay:
type A3 = Awesome<A, 'a', number>
type A4 = Awesome<A, 'b', string>

// not okay:
type A5 = Awesome<A, 'a', boolean>
type A6 = Awesome<A, 'b', Date>

Playground.

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编辑

正如 Daniel Rosenwasser 指出的那样，keyof 称为“键查询”，ObjType[KeyType] 是“索引访问类型”。