enum-annotations.js

/* @flow */

// The following three lines are from https://gist.github.com/nmn/b20a92e848c68d88f7fdb8353c71b1ca
type $Object<V> = { +[ key : string] : V }
type $ObjectValues<V, O : $Object<V>> = V
type $Values<O: Object> = $ObjectValues<*, O>

type Enumerables = { [ key : string ] : any }

const UserRoles : Enumerables = {
	ADMIN: 1,
	BASIC: 2,
	MOD: 'MODERATOR',
	BOT: 'I_R_A_PROGRAMZ'
}

type UserRole = $Values<typeof UserRoles>

class User {

	currentRole : UserRole = UserRoles.BASIC

	constructor( userRole : ?UserRole ) : void {
		this.role = userRole
		
	}

	set role( newUserRole : ?UserRole ) : void {
		if ( newUserRole ) {
			this.currentRole = newUserRole
		}
	}

	get role() : any {
		// @NOTE Mind the any return type, as defined in Enumerables.
		return this.currentRole;
	}

}

const basicUser = new User()
console.log( basicUser.role === UserRoles.BASIC )

// Alter just to test it out
basicUser.role = UserRoles.MOD
console.log( basicUser.role === UserRoles.MOD )

const botUser = new User(UserRoles.BOT)
console.log( botUser.role === UserRoles.BOT )

// @NOTE Magic

Caveats
basicUser.role = 'some arbitrary value' doesn't really trigger an error, flow annotations are in fact valid, so this is expected. Could be easily fixed by having a simple enum-validator in set role.

const ValidUserRole : Function = ( expectedUserRole : any ) : Bool => Object.values(UserRoles).includes(expectedUserRole)

class User {

  set role( newUserRole : ?UserRole ) : void {
    if ( newUserRole !== undefined && ValidUserRole(newUserRole) ) {
      this.currentRole = newUserRole
    }
  }

}