Typescript: string literal union type from enum

TypeScript 4.1+:

As mentioned, this can be achieved by using Template Literal Types like so:

type WeekdayType = `${Weekday}`;

TypeScript 3.4+:

Following up on @jcalz answer and the comment from @just-boris, here's an example with const assertions:

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

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

Edit:

Wrote a blog post for those who would like to dig deeper.

See TS4.1 ANSWER:

type WeekdayType = `${Weekday}`;

---

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.

---

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!