Typescript: string literal union type from enum

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

See TS4.1 ANSWER:

type WeekdayType = `${Weekday}`; 

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PRE TS-4.1 ANSWER:

This can't be done programmatically... you're trying to convert the type Weekday, which is Weekday.MONDAY | Weekday.TUESDAY | Weekday.WEDNESDAY, to the type WeekdayType which is "mon" | "tue" | "wed". This conversion is a form of widening, since Weekday is a subtype of WeekdayType:

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

Unfortunately the compiler doesn't give you a handle to remove an "enum"-ness from the enum type and leave you with plain literal types.

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So, workarounds? Maybe you don't actually need an enum, but can make do with an object whose property values are string literals:

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], 

If you inspect it, the value named Weekday behaves like an enum object:

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

while the type named Weekday behaves like the union of string values "mon" | "tue" | "wed" that you were calling WeekdayType:

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

So in this workaround, there is no "enum"-ness, and therefore no need to distinguish the type Weekday from the type WeekdayType. It's a little different from an actual enum (which includes types like Weekday.MONDAY, which you'd have to represent as the cumbersome typeof Weekday.MONDAY or create a different type alias for it), but it might behave similarly enough to be useful. Does that work for you?

Hope that helps. Good luck!