Discriminated Unions für Component-Varianten

type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'rectangle'; width: number; height: number };

function area(shape: Shape): number {
  if (shape.kind === 'circle') return Math.PI * shape.radius ** 2;
  return shape.width * shape.height; // TS knows width/height exist here
}

type ButtonProps =
  | { as: 'button'; onClick: () => void; disabled?: boolean; children: React.ReactNode }
  | { as: 'a'; href: string; target?: string; children: React.ReactNode };

function ActionButton(props: ButtonProps) {
  if (props.as === 'button') {
    return <button onClick={props.onClick} disabled={props.disabled}>{props.children}</button>;
  }
  return <a href={props.href} target={props.target}>{props.children}</a>;
}

type AlertProps =
  | { type: 'success'; message: string }
  | { type: 'error'; message: string; onRetry: () => void }
  | { type: 'warning'; message: string; details?: string };

function Alert(props: AlertProps) {
  if (props.type === 'error') {
    return (
      <div className="alert error">
        <p>{props.message}</p>
        <button onClick={props.onRetry}>Retry</button>
      </div>
    );
  }
  return <div className={`alert ${props.type}`}>{props.message}</div>;
}

type LoadingState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

function DataView<T>({ state }: { state: LoadingState<T> }) {
  if (state.status === 'loading') return <Spinner />;
  if (state.status === 'error') return <p>{state.error.message}</p>;
  if (state.status === 'success') return <pre>{JSON.stringify(state.data)}</pre>;
  return null;
}

function assertNever(x: never): never {
  throw new Error('Unhandled case: ' + x);
}

function renderIcon(type: AlertProps['type']) {
  switch (type) {
    case 'success': return <CheckIcon />;
    case 'error': return <XIcon />;
    case 'warning': return <WarnIcon />;
    default: return assertNever(type); // TS error if a case is missing
  }
}

type ApiResult<T> =
  | { ok: true; data: T }
  | { ok: false; error: string; code: number };

async function fetchUser(id: string): Promise<ApiResult<User>> {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) return { ok: false, error: 'Not found', code: res.status };
  return { ok: true, data: await res.json() };
}

function isSuccess<T>(result: ApiResult<T>): result is { ok: true; data: T } {
  return result.ok === true;
}

const result = await fetchUser('1');
if (isSuccess(result)) {
  console.log(result.data.name); // TS knows data exists
}

// Bad: optional prop soup — invalid combos allowed:
interface BadProps {
  href?: string;
  onClick?: () => void;
  disabled?: boolean;
}

// Good: only valid combos via discriminated union:
type GoodProps =
  | { as: 'a'; href: string }
  | { as: 'button'; onClick: () => void; disabled?: boolean };

type BaseProps = { className?: string; children: React.ReactNode };

type ButtonVariant =
  | (BaseProps & { variant: 'primary'; onClick: () => void })
  | (BaseProps & { variant: 'link'; href: string });