Differentiate between different Strings in TypeScript

Question

I have a set of Types which are just strings, ie.

type FooId = string
type BarId = string

const callFoo = (id: FooId) => {}

const b: BarId = 'bar'

callFoo(b)

To me, that should error, because I'm passing a BarID when I need to pass a FooID

How can I tell TypeScript to give special meaning to these, essentially, strings?

More context:

import { monotonicFactory } from "ulid";

export type ULID = string;
export const ulid = monotonicFactory() as () => ULID;

type FooId = ULID
type BarId = ULID

They're all ULID's, not just "strings"

Answer 1

Let's utilize @basarat's TDD book: Nominal Typing

Because you are using strings here, I think the enum method is the best choice:

enum FooBrand { _ = "" }
enum BarBrand { _ = "" }

type FooId = string & FooBrand
type BarId = string & BarBrand

const callFoo = (id: FooId) => {}

const b = 'bar' as BarId

callFoo(b); // now errors

You can read more about other nominal typing methods in the link above.

Playground demo