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?

@jcalz when people 'answer' in comments like you did, the answers cannot be edited by the community. It's a pity, as this question can now be solved, but people coming onto this question will first read your (then correct, now incorrect) comment and may be mislead as a result. — Steve Dodier-Lazaro, Aug 19 '23 at 09:37

score 191 · Accepted Answer · edited Dec 24 '22 at 20:54

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?

Update: since typescript 3.4, special `lit` function is no longer needed. You can use [const assertions](https://devblogs.microsoft.com/typescript/announcing-typescript-3-4/#const-assertions) instead — just-boris, Sep 22 '19 at 20:26
curios question @jcalz. What does `` do? it prevents widening any kind of string/number literal ? thanks! — hotell, Aug 09 '20 at 09:24
is there a way to do the opposite? Get a string enum from a type that is a union of string literals? — Pablo K, May 11 '21 at 00:39

designcise · Answer 2 · 2021-09-20T01:49:41.007

69

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.

edited Sep 20 '21 at 01:49

answered Dec 27 '19 at 04:58

designcise

4,204
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1

omigod thank you. I was flailing for hours trying to figure this out. – Seth Bro Oct 24 '20 at 00:47
That should work but has a limitation. ``` type Weekday1 = (typeof Weekday)[keyof typeof Weekday]; type Weekday2 = `${Weekday}`; const day1: Weekday1 = Weekday.MONDAY; => works const day2: Weekday2 = Weekday.MONDAY; => works const day3: Weekday1 = 'mon' => does not work const day4: Weekday2 = 'mon'; => works ``` – Mina Luke Jan 04 '21 at 22:30

coyotte508 · Answer 3 · 2022-11-27T22:05:19.283

52

With Typescript 4.1, it can be done!

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

type WeekdayType = `${Weekday}`;

And for number enums, thanks to @okku:

enum ThreeDigits {
  ZERO = 0,
  ONE = 1,
  TWO = 2
}

type ThreeDigitsType = `${ThreeDigits}` extends `${infer T extends number}` ? T : never;

edited Nov 27 '22 at 22:05

answered Nov 23 '20 at 10:31

coyotte508

9,175
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4

An easier way to to this is: type WeekdayUnion = `${Weekday}`; That's it! – Nonoroazoro Nov 27 '20 at 08:21
1

any luck getting this to compile with ts-node? – Michael G Dec 16 '20 at 04:59
2

@MichaelG yes, you just need to update your typescript version to 4.1 in your project's package.json in your dev dependencies – coyotte508 Dec 17 '20 at 20:09
3

Could someone provide documentation for this behaviour plz?! – sandbox992 May 30 '21 at 15:09
Wish this didn't convert numbers to strings, but makes sense, given that it's overloading string interpolation syntax – Devin Rhode Apr 07 '22 at 00:55
what if the values are all numbers ? – Jason Wong Jul 15 '22 at 01:22
1

It does work with number enums, with some extra work: `type WeekdayType = `${Weekday}` extends `${infer T extends number}` ? T : never;` – Okku Nov 27 '22 at 04:09

Typescript: string literal union type from enum

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