typescript - 作为映射键的字符串枚举不适用于 ReturnType

Question

我正在尝试为functions. 以下代码有效。

enum Enum {
  Foo = "foo"
}

const functions = {
  [Enum.Foo]: () => "Hello World!"
};

type FunctionMap = { [key: string]: (...args: any[]) => any };
type FunctionUnion<T extends FunctionMap> = ReturnType<T[keyof T]>;

type functions = FunctionUnion<typeof functions>;

但是，如果我更改 的键类型FunctionMap，

type FunctionMap = { [key in Enum]: (...args: any[]) => any };

它抛出

类型 'T[keyof T]' 不满足约束 '(...args: any[]) => any'。类型 'FunctionMap[keyof T]' 不可分配给类型 '(...args: any[]) => any'.ts(2344)

为什么会抛出错误？

标签： typescript

Answer 1

您会收到枚举错误，因为索引签名[key: string]意味着任何可以存在的属性都FunctionMap必须是 type (...args: any[]) => any。Enum另一方面，映射类型意味着枚举中存在的属性必须是函数，该类型不保证其他属性。所以这是有效的：

enum Enum {
  Foo = "foo"
}

const functions = {
  [Enum.Foo]: () => "Hello World!",
  otherProp: "string"
};

type FunctionMap = { [key in Enum]: (...args: any[]) => any };
type FunctionUnion<T extends FunctionMap> = ReturnType<T[keyof T]>;
type functions = FunctionUnion<typeof functions>; //ok  here otherProp is not a problem

一种选择是使用条件类型过滤掉非函数类型：

enum Enum {
  Foo = "foo"
}

const functions = {
  [Enum.Foo]: () => "Hello World!",
  otherProp: "string"
};

type FunctionMap = { [key in Enum]: (...args: any[]) => any };
type FunctionUnion<T extends FunctionMap> = { 
    [P in keyof T]: T[P] extends (...args: any[]) => any ? ReturnType<T[P]> : never;
}[keyof T]
type functions = FunctionUnion<typeof functions>;