Amarga TypeScript nggunakake tipe struktural, UserId lan OrderId sing loro-lorone string bisa digatekake kanthi saka - compiler ora bisa megegem sliramu saka nyempur-sempur. Branded types nyimulasi nominal typing kanthi nambahake unika, phantom "brand" supaya tipe sing padha dadi beda.
// 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
Properti __brand ora tau ana ing runtime (nilainya mung string), nanging ing sistem tipe ndadekake UserId lan OrderId ora kompatibel - nyegah kabeh kelas bug nyempur.
function toUserId(s: string): UserId {
if (!s.startsWith("u")) throw new Error("invalid UserId");
return s as UserId; // validation happens here, once
}
Validasi sapisan ing wates; sisa kode dudu branded type.
IDs (UserId vs OrderId), string sing divalidasi/disanitasi (Email, SafeHtml), unit (Meters vs Feet), PositiveNumber - endi wae loro nilai ranggo tipe primitif nanging kudu ora kaliru.
Branded types nambahake nominal safety endi structural typing asale permissive banget.
Dheweke nyekel argument-order mix-ups lan "unvalidated vs validated" kabingungan ing wektu compile, kanthi zero runtime cost - pola apik kanggo domain modeling ing besar, codebases sing kritis korektness.