Объясните narrowing и type guards в TypeScript: что это, как работает и когда применять.

// Built-in narrowing follows control flow — no casts needed
function fmt(x: string | number | null) {
  if (x == null) return 'none';          // narrows out null AND undefined
  if (typeof x === 'string') return x.trim(); // x: string here
  return x.toFixed(2);                    // x: number here
}

// ❌ typeof object includes null (the JS legacy bug)
function bad(x: object | null) {
  if (typeof x === 'object') x.valueOf(); // ❌ x could still be null!
}

// ✅ user-defined type guard — reusable narrowing (but UNVERIFIED)
function isString(x: unknown): x is string {
  return typeof x === 'string'; // TS trusts this return; it does NOT check it
}

// ✅ assertion function — narrows by throwing
function assert(cond: unknown, msg: string): asserts cond {
  if (!cond) throw new Error(msg);
}
function use(x: string | undefined) {
  assert(x, 'missing'); // after this line, x is string
  return x.length;
}

// ❌ A lying guard type-checks — the runtime hole
function isUser(x: unknown): x is { id: number } { return true; }
const u = JSON.parse('null');
if (isUser(u)) u.id; // compiles; crashes — the guard body was a lie