TypeScript: типизация async и generics в API — пример кода

// ❌ any leaks everywhere — the return is unusable-safe
async function get(url: string) {
  const res = await fetch(url);
  return res.json(); // Promise<any> → poisons every caller silently
}

// ⚠️ Better: generic threads the type, but it's an unchecked assertion
async function getJSON<T>(url: string): Promise<T> {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`HTTP ${res.status} for ${url}`);
  return res.json() as Promise<T>;
}

// ✅ Best: constrained generic + runtime validation via an injected parser
async function getValidated<T>(
  url: string,
  parse: (raw: unknown) => T,   // the caller supplies the runtime check
): Promise<T> {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return parse(await res.json()); // type AND runtime shape now agree
}

// A discriminated Result type — model failure in the type, not just throws
type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };

async function safe<T>(p: Promise<T>): Promise<Result<T>> {
  try { return { ok: true, value: await p }; }
  catch (e) { return { ok: false, error: e as Error }; }
}

const r = await safe(getJSON<User>("/api/user"));
if (r.ok) r.value.name;   // ✅ narrowed to User
else r.error.message;     // ✅ narrowed to Error