Use Const Values As Union Type Options Typescript

Consider this example: const FIRST = 'FIRST' const SECOND = 'SECOND' type TOptions = FIRST | SECOND const someFunction = (options: TOptions): void => { // do something, no

Solution 1:

I believe you need to use typeof to make it work:

const FIRST = 'FIRST'
const SECOND = 'SECOND'

type TOptions = typeof FIRST | typeof SECOND;

const someFunction = (options: TOptions): void => {
// do something, no return
}

someFunction(FIRST); // works
someFunction('test'); // error

Playground Link

Solution 2:

You can declare those constants as types:

// It's a good idea to export these types 
// since you want them to be used external files
export type FIRST = "FIRST"
export type SECOND = "SECOND"

export type TOptions = FIRST | SECOND

// and then a call to your function will be
someFunction("FIRST") // ok
someFunction("SECOND") // ok
someFunction("THIRD") // Argument of type '"THIRD"' is not assignable to parameter of type 'TOptions'.(2345)

Another option is to use an enum type:

export enum Options {
  FIRST = 'FIRST',
  SECOND = 'SECOND'
}

const otherFunction = (options: Options) => {}

otherFunction(Options.FIRST)
otherFunction(Options.SECOND)