TypeScript: discriminated unions — пример кода

// ✅ Model state as a discriminated union — impossible states can't exist
type RequestState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }        // data ONLY exists here
  | { status: 'error'; error: Error };    // error ONLY exists here

function render(s: RequestState<string>) {
  switch (s.status) {
    case 'idle':    return 'Start';
    case 'loading': return 'Spinner';
    case 'success': return s.data;         // ✅ data is available & typed
    case 'error':   return s.error.message;
    default: {
      const _exhaustive: never = s;        // ✅ compile error if a case is added & unhandled
      return _exhaustive;
    }
  }
}

// ❌ The bag-of-flags anti-pattern — permits impossible states
type Bad = { loading: boolean; data?: string; error?: Error };
const b: Bad = { loading: true, data: 'x', error: new Error() }; // nonsense, but legal

// Narrowing needs a LITERAL discriminant
type Ok = { kind: 'a'; x: number } | { kind: 'b'; y: string };
declare let k: string;
// if (shape.kind === k) — ❌ won't narrow; k is widened `string`, not a literal