How to define string literal union type from constants in Typescript

Question

I know I can define string union types to restrict variables to one of the possible string values:

type MyType = 'first' | 'second'
let myVar:MyType = 'first'

I need to construct a type like that from constant strings, e.g:

const MY_CONSTANT = 'MY_CONSTANT'
const SOMETHING_ELSE = 'SOMETHING_ELSE'
type MyType = MY_CONSTANT | SOMETHING_ELSE

But for some reason it doesn't work; it says MY_CONSTANT refers to a value, but it is used as a type here.

Why does Typescript allow the first example, but doesn't allow the second case? I'm on Typescript 3.4.5

Answer 1

You can also use enum for this case. For example:

// Define enum.
enum myConstants {
  MY_CONSTANT = 'my_constant',
  SMTH_ELSE = 'smth_else'
}

// Use it in an interface for typechecking.
interface MyInterface {
  myProp: myConstants
}

// Example of correct object - no errors.
let a: MyInterface = {
  myProp: myConstants.MY_CONSTANT
}

// Incorrect value - TS reports an error.
let b: MyInterface = {
  myProp: 'John Doe'
}