Make type from enum's number values in Typescript

Question

Is it possible to convert enum that looks for exmp:

enum StatusCodeEnum {
  OK = 200,
  BAD_REQUEST = 400,
  UNAUTHORIZED = 401,
  FORBIDDEN = 403,
  NOT_FOUND = 404,
  INTERNAL_SERVER_ERROR = 500,
  SERVICE_UNAVAILABLE = 503,
}

to template type that will have only numbers like

TypeFromEnum = 200 | 400 | 500 | ...  ---> numbers

i only interested in enum transformation without switching to plain Object with "as const"

Answer 1

This will be possible in TypeScript 4.8, with more specific inference from template string types.

The approach looks like this:

type TypeFromEnum = `${Extract<StatusCodeEnum, number>}` extends 
  `${infer N extends number}` ? N : never;
// type TypeFromEnum = 200 | 400 | 401 | 403 | 404 | 500 | 503

You can't easily iterate through a purely numeric enum because it's not internally represented as a union type, so the method above first makes sure it grabs only numeric enum members (in case it's a mixed string/numeric enum), converts them to a union of string literal types, and then converts back into numeric literal types via the infer N extends nmber technique.

Playground link to code

Answer 2

try this:

type StatusCodeType = `${StatusCodeEnum}`

// return -> `200 | 400 | 401 | ... `

Answer 3

//can use this if you only want specific enum values

type typeFromEnum = StatusCodeEnum.OK | StatusCodeEnum.BAD_REQUEST | StatusCodeEnum.INTERNAL_SERVER_ERROR

// Or this if you want the type to be able to be any of the enum but you might as well just use the enum as the type.
type typeFromEnum = StatusCodeEnum;