React JS Interview Questions in Kolkata

Create a wrapper that checks auth and either renders the child routes or redirects. import { Navigate, Outlet, useLocation } from 'react-router'; function RequireAuth() { const { user } = useAuth(); const location = useLocation(); if (!user) return <Navigate to="/login" replace state={{ from: location }} />; return <Outlet />; } <Route element={<RequireAuth />}> <Route path="/account" element={<Account />} /> </Route> The server must still enforce authorisation.

State updates are scheduled, and each render sees a snapshot of state. If you call setCount(count + 1) several times in one handler, all calls read the same old value. The updater function receives the latest pending value. function addThree() { setCount(c => c + 1); setCount(c => c + 1); setCount(c => c + 1); } // count increases by 3, not 1

Define a schema and pass it through zodResolver. const schema = z.object({ email: z.string().email('Invalid email'), password: z.string().min(8, 'Min 8 characters') }); function Login() { const { register, handleSubmit, formState: { errors, isSubmitting } } = useForm({ resolver: zodResolver(schema) }); return ( <form onSubmit={handleSubmit(data => login(data))}> <input {...register('email')} /> {errors.email && <p>{errors.email.message}</p>} <input type="password" {...register('password')} /> <button disabled={isSubmitting}>Login</button> </form> ); }

A custom hook is a function starting with use that combines other hooks. function useFetch(url) { const [state, setState] = useState({ data: null, loading: true, error: null }); useEffect(() => { const ctrl = new AbortController(); setState(s => ({ ...s, loading: true })); fetch(url, { signal: ctrl.signal }) .then(r => { if (!r.ok) throw new Error(r.status); return r.json(); }) .then(data => setState({ data, loading: false, error: null })) .catch(err => { if (err.name !== 'AbortError') setState({ data: null, loading: false, error: err }); }); return () => ctrl.abort(); }, [url]); return state; }

Zustand is a small state library with a hook-based API and no Provider. import { create } from 'zustand'; const useCart = create(set => ({ items: [], add: item => set(s => ({ items: [...s.items, item] })), clear: () => set({ items: [] }) })); const count = useCart(s => s.items.length); Components subscribe with selectors, so they re-render only when the selected slice changes.

TanStack Query caches by query key and handles loading, errors, retries and background refetching. const queryClient = new QueryClient(); // wrap app: <QueryClientProvider client={queryClient}> function Orders({ userId }) { const { data, isPending, error } = useQuery({ queryKey: ['orders', userId], queryFn: () => fetch(`/api/users/${userId}/orders`).then(r => r.json()), staleTime: 60_000 }); if (isPending) return <Spinner />; if (error) return <p>Error</p>; return <OrderList orders={data} />; }

Responses can arrive out of order, so a slow old request overwrites newer results. Cancel or ignore stale requests in the effect cleanup. useEffect(() => { const ctrl = new AbortController(); fetch(`/api/search?q=${q}`, { signal: ctrl.signal }) .then(r => r.json()) .then(setResults) .catch(() => {}); return () => ctrl.abort(); }, [q]); Also debounce the input, or use TanStack Query which handles this for you.

Make the page an async Server Component and await data directly. // app/products/[id]/page.tsx export default async function ProductPage({ params }) { const { id } = await params; const product = await getProduct(id); if (!product) notFound(); return ( <main> <h1>{product.name}</h1> <AddToCartButton id={id} /> </main> ); } AddToCartButton is a Client Component. In recent Next.js versions params is a promise.

Use vi.mock with a factory, then control return values per test. import { vi } from 'vitest'; import { getProfile } from './api'; vi.mock('./api', () => ({ getProfile: vi.fn() })); test('shows name', async () => { getProfile.mockResolvedValue({ name: 'Ravi' }); render(<Profile />); expect(await screen.findByText('Ravi')).toBeInTheDocument(); }); In Jest, the same pattern uses jest.mock and jest.fn.

Define a type or interface and annotate the destructured props. type ProductCardProps = { title: string; price: number; tags?: string[]; onAdd: (id: string) => void; children?: React.ReactNode; }; function ProductCard({ title, price, tags = [], onAdd, children }: ProductCardProps) { return <div onClick={() => onAdd(title)}>{title} - Rs {price}{children}</div>; }