TypeScript-Lookup Types - Cannot access ‘xxx‘ because ‘xxx‘ is a type, but not a namespac

布满荆棘的人生 2022-10-28 14:26 25阅读 0赞

如果我想希望使用某种类型声明的子类型作为某个变量声明的类型…

export interface Car { 
        Name: string;
        Speed: number;
        Manufactured: number;
    }
    
    const Speed: Car.Speed = 200;

上面的写法会抛出错误

> Cannot access ‘Car.Speed’ because ‘Car’ is a type, but not a namespace. Did you mean to retrieve the type of the property ‘Speed’ in ‘Car’ with ‘Car\[“Speed”\]’

这是因为你的使用姿势不对。需要用如下的写法

const Speed: Car['Speed'] = 200;

这种类型称为`Lookup Types`

这种使用某种类型的"成员类型"作为类型声明的使用有两种方式：

interface Person { 
      name: string;
      age: number;
      location: string;
    }
    
    type K1 = keyof Person; // "name" | "age" | "location"
    type K2 = keyof Person[]; // "length" | "push" | "pop" | "concat" | ...
    type K3 = keyof {  [x: string]: Person }; // string

或者

type P1 = Person["name"]; // string
    type P2 = Person["name" | "age"]; // string | number
    type P3 = string["charAt"]; // (pos: number) => string
    type P4 = string[]["push"]; // (...items: string[]) => number
    type P5 = string[][0]; // string

参考：

[https://stackoverflow.com/questions/55607608/how-can-i-reuse-a-type-of-an-interface-member][https_stackoverflow.com_questions_55607608_how-can-i-reuse-a-type-of-an-interface-member]

[https_stackoverflow.com_questions_55607608_how-can-i-reuse-a-type-of-an-interface-member]: https://stackoverflow.com/questions/55607608/how-can-i-reuse-a-type-of-an-interface-member