Brand- eli opaakit tyypit yksikköjen sekoittumisen estämiseen

type Meter = number & { readonly __brand: "meter" }
type Second = number & { readonly __brand: "second" }

function meter(n: number): Meter { return n as Meter }
function second(n: number): Second { return n as Second }

const d = meter(100)
const t = second(9)

function speed(dist: Meter, time: Second) { return (dist as number) / (time as number) }

// speed(t, d)  // compile error: brands do not match

type Brand<Tag extends string, T> = T & { readonly __brand: Tag }

type UserId = Brand<"UserId", string>
type OrderId = Brand<"OrderId", string>

function userId(x: string): UserId { return x as UserId }
function orderId(x: string): OrderId { return x as OrderId }

function findUser(id: UserId) { /* ... */ }

const u = userId("u_123")
const o = orderId("o_456")
// findUser(o) // error: OrderId not assignable to UserId

declare const MeterSym: unique symbol

type Meter2 = number & { readonly [MeterSym]: "meter" }

function asMeter2(n: number): Meter2 { return n as Meter2 }

const m2 = asMeter2(5)
// const wrong: Meter2 = 5  // discourage raw assignment; prefer factories

type NonEmptyString = string & { readonly __brand: "NonEmpty" }

function nonEmpty(s: string): NonEmptyString | null {
  return s.trim().length > 0 ? (s as NonEmptyString) : null
}

function makeSlug(s: NonEmptyString) {
  return (s as string).toLowerCase().replace(/\s+/g, "-")
}

const ok = nonEmpty(" Hello ")
if (ok) { makeSlug(ok) }  // safe: validated & branded first