Kwa sababu TypeScript ni structurally typed, UserId na OrderId ambazo zote ni string zinaweza kubadilishwa — compiler haiwezi kukuzuia kutokusanya. Branded types husimulisha nominal typing kwa kuambatanisha brand ya kipekee, phantom ili aina zinazofanana ziwe tofauti.
// the brand is a property that exists only in the type system, never at runtime
type Brand<T, B> = T & { readonly __brand: B };
type UserId = Brand<string, "UserId">;
type OrderId = Brand<string, "OrderId">;
function getUser(id: UserId) { return id; }
const uid = "u1" as UserId;
const oid = "o1" as OrderId;
getUser(uid); // ✅
getUser(oid); // ❌ Error: OrderId is not assignable to UserId
getUser("u1"); // ❌ Error: plain string is not a UserId
Sifa ya __brand haipo wakati wa runtime (thamani ni string tu), lakini katika sistemu ya aina inafanya UserId na OrderId kuwa hazilingani — kinachozuia darasa lote la matatizo ya uchanganyaji.
function toUserId(s: string): UserId {
if (!s.startsWith("u")) throw new Error("invalid UserId");
return s as UserId; // validation happens here, once
}
Thibitisha mara moja kwa mpaka; nambari iliyobaki ya code inaaminisha aina ya branded.
IDs (UserId vs OrderId), validated/sanitized strings (Email, SafeHtml), units (Meters vs Feet), PositiveNumber — popote thamani mbili zinashiriki aina ya primitive lakini lazima zisikataane.
Branded types huongeza usalama wa nominal ambapo structural typing ni penyevu sana.
Zinakamata kosa la mpangilio wa argument na "unvalidated vs validated" utata wakati wa kukamatia, bila gharama yoyote ya runtime — mfumo wenye nguvu kwa domain modeling katika codebases kubwa, zenye umuhimu wa usahihi.