"请考虑使用映射对象类型." - 什么是映射对象类型,我如何在这里使用它？

Question

interface Foo { 
    [foo: "hello" | "world"]: string;
}

我收到一条错误消息

An index signature parameter type cannot be a union type. Consider using a mapped object type instead.

什么是映射对象类型,以及如何使用它？

Answer 1

映射对象类型对一组单例类型进行操作,并生成一个新的对象类型,其中每个单例都转换为属性名称.

例如,这个:

type Foo = {
    [K in "hello" | "world"]: string
};

相当于

type Foo = {
    "hello": string;
    "world": string;
};

请记住,映射的对象类型是一个不同的类型操作符 - 大括号中的语法不能用于接口,也不能用于与其他成员的对象类型.例如

interface Foo {
    [K in "hello" | "world"]: string
}

产生以下错误:

A computed property name in an interface must refer to an expression whose type is a literal type or a 'unique symbol' type.

映射的对象类型对许多不同的东西都很有用.在这里阅读更多内容:http://www.typescriptlang.org/docs/handbook/advanced-types.html#mapped-types

Answer 2

这是无效的：

    type DirectiveType = 'package-as' | 'externals' | 'move-folders' | 'ignore';
    type Directives = { [key:DirectiveType]?: RootDirectives };
    
    export class PkgmetaFile {
        private _directives: Directives = {};
    }

但这是有效的：

    type DirectiveType = 'package-as' | 'externals' | 'move-folders' | 'ignore';
    type Directives = { [Key in DirectiveType as string]?: RootDirectives };
    
    export class PkgmetaFile {
        private _directives: Directives = {};
    }