“考虑使用映射对象类型。”什么是映射对象类型?我在这里如何使用它?

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:

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独特的类型运算符——大括号中的语法不能用于接口,也不能用于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.

映射对象类型对于许多不同的事情都很有用。请在此处阅读更多内容: http://www.typescriptlang.org/docs/handbook/advanced-types.html#mapped-types

这是无效的:

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

但这是正确的:

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