Full Stack with Node.js Interview Questions in Noida

const ALLOWED_SORT = ['price', 'createdAt', 'name']; router.get('/products', async (req, res) => { const filter = {}; if (req.query.category) filter.category = String(req.query.category); if (req.query.maxPrice) filter.price = { $lte: Number(req.query.maxPrice) }; const field = ALLOWED_SORT.includes(req.query.sort) ? req.query.sort : 'createdAt'; const dir = req.query.order === 'asc' ? 1 : -1; res.json(await Product.find(filter).sort({ [field]: dir }).limit(50)); }); // Whitelist fields and cast types to block injection.

const { z } = require('zod'); const createUser = z.object({ name: z.string().min(2), email: z.string().email(), age: z.number().int().min(18).optional() }); const validate = (schema) => (req, res, next) => { const r = schema.safeParse(req.body); if (!r.success) return res.status(400).json({ errors: r.error.issues }); req.body = r.data; next(); }; router.post('/users', validate(createUser), createUserHandler);

const fs = require('fs'); const readline = require('readline'); async function countErrors(path) { const rl = readline.createInterface({ input: fs.createReadStream(path), crlfDelay: Infinity }); let count = 0; for await (const line of rl) { if (line.includes('ERROR')) count++; } return count; } // Streams line by line, so a 5 GB log uses little memory.

router.get('/products', async (req, res) => { const limit = Math.min(Number(req.query.limit) || 20, 100); const cursor = req.query.cursor; const filter = cursor ? { _id: { $lt: cursor } } : {}; const items = await Product.find(filter) .sort({ _id: -1 }) .limit(limit + 1); const hasMore = items.length > limit; if (hasMore) items.pop(); res.json({ data: items, nextCursor: hasMore ? items[items.length - 1]._id : null }); });

// order.service.test.js jest.mock('../repositories/orderRepo'); const repo = require('../repositories/orderRepo'); const { cancelOrder } = require('../services/order.service'); test('cannot cancel a shipped order', async () => { repo.findById.mockResolvedValue({ id: 1, status: 'shipped' }); await expect(cancelOrder(1)).rejects.toThrow('Cannot cancel'); expect(repo.update).not.toHaveBeenCalled(); }); // For HTTP calls, nock or msw can intercept requests.

class AppError extends Error { constructor(message, status = 500) { super(message); this.status = status; } } app.use((err, req, res, next) => { const status = err.status || 500; logger.error({ err, path: req.path }); res.status(status).json({ error: status === 500 ? 'Internal server error' : err.message }); }); // In a route: throw new AppError('User not found', 404);

const asyncHandler = (fn) => (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next); router.get('/:id', asyncHandler(async (req, res) => { const user = await User.findById(req.params.id); if (!user) throw new AppError('Not found', 404); res.json(user); })); // Without this, a rejected promise is never sent to the error handler. // Express 5 does this automatically.

function withTimeout(promise, ms) { let timer; const timeout = new Promise((_, reject) => { timer = setTimeout(() => reject(new Error('Timeout')), ms); }); return Promise.race([promise, timeout]).finally(() => clearTimeout(timer)); } await withTimeout(fetch(url), 3000); // For fetch, AbortSignal.timeout(3000) also cancels the request.