TypeScript Types and Interfaces

interface User { id: string; email: string; name?: string; // optional readonly createdAt: Date; } interface AdminUser extends User { role: "admin"; } const u: AdminUser = { id: "u_1", email: "ada@example.com", createdAt: new Date(), role: "admin", }; // u.createdAt = new Date(); // error: readonly

type ID = string & { readonly __brand: "ID" }; // branded nominal type type Result<T> = | { ok: true; value: T } | { ok: false; error: string }; function parse(input: string): Result<number> { const n = Number(input); return Number.isNaN(n) ? { ok: false, error: "not a number" } : { ok: true, value: n }; } const r = parse("42"); if (r.ok) console.log(r.value * 2); else console.error(r.error);