Question

I'd like to get a string literal union from an enum.

For this enum…

enum Weekday {
    MONDAY = 'mon',
    TUESDAY = 'tue',
    WEDNESDAY = 'wed'
}

… I'd like to get this:

type WeekdayType = 'mon' | 'tue' | 'wed';

I tried typeof keyof Weekday but that resulted in 'MONDAY' | 'TUESDAY' | 'WEDNESDAY'. Feel like the solution might have to do with mapped types but I can't seem to wrap my head around it.

How do I do this?

Answer 1

这无法通过编程方式完成...您正在尝试将Weekday类型Weekday.MONDAY | Weekday.TUESDAY | Weekday.WEDNESDAY转换为WeekdayType类型"mon" | "tue" | "wed" 。由于Weekday是WeekdayType的子类型，因此此转换是扩大的一种形式：

type WeekdayExtendsWeekdayType = 
  Weekday extends WeekdayType ? true : false
// type WeekdayExtendsWeekdayType = true

不幸的是，编译器没有给您提供从枚举类型中删除“枚举”性质并使您拥有普通文字类型的句柄。

那么，解决方法？也许您实际上并不需要enum，但可以处理其属性值为字符串文字的对象：

const lit = <V extends keyof any>(v: V) => v;
const Weekday = {
  MONDAY: lit("mon"),
  TUESDAY: lit("tue"),
  WEDNESDAY: lit("wed")
}
type Weekday = (typeof Weekday)[keyof typeof Weekday],

如果进行检查，则名为Weekday的 value 的行为类似于枚举对象：

console.log(Weekday.TUESDAY); // tue

名为Weekday的 type 的行为类似于您调用"mon" | "tue" | "wed"的字符串值WeekdayType的并集：

const w: Weekday = "wed"; // okay
const x: Weekday = "xed"; // error

因此，在此替代方法中，没有“枚举”性质，因此无需将类型Weekday与类型WeekdayType区分开。与实际的enum（包括Weekday.MONDAY之类的类型，您必须将其表示为笨重的typeof Weekday.MONDAY或创建其他类型）有所不同别名），但它的行为可能也足够有用。这样对您有用吗？

希望有帮助。祝你好运！

Answer 2

使用Typescript 4.1，可以做到！

enum Weekday {
  MONDAY = 'mon',
  TUESDAY = 'tue',
  WEDNESDAY = 'wed'
}

type WeekdayType = `${Weekday}`;

注意：仅适用于字符串枚举值，不适用于数字枚举值。

Answer 3

紧接着@jcalz答案和@ just-boris的评论，下面是一个带有const断言（打字稿3.4+）的示例：

const Weekday = {
  MONDAY: "mon",
  TUESDAY: "tue",
  WEDNESDAY: "wed",
} as const;

type Weekday = (typeof Weekday)[keyof typeof Weekday],

Typescript: string literal union type from enum

3 个答案: