36 practice questions with answers for MERN Stack interviews in Jaipur.
MERN Stack jobs in Jaipur
Jaipur has a growing IT and IT-enabled services market, with delivery centres, SaaS start-ups and back-office analytics teams around Sitapura and Mahindra World City. Many candidates here also apply to remote and hybrid roles with Delhi NCR teams.
Below are 36 questions that match the mix of service, product and domain employers hiring mern stack developers in Jaipur.
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01
Write a Supertest integration test for a POST /api/users endpoint.
Redux keeps global state in a single store that changes only through dispatched actions handled by reducers. Redux Toolkit is the official modern way to write Redux: configureStore sets up the store with DevTools, createSlice generates actions and reducers and lets you write mutating code safely using Immer, and createAsyncThunk handles async requests. RTK Query adds data fetching and caching. Components use useSelector and useDispatch.
03
Attackers are trying thousands of passwords on your login endpoint. How do you defend it?
Apply strict rate limits on login per IP and per account using a shared Redis store, add progressive delays or temporary account lockout after several failures, and show a CAPTCHA after repeated attempts. Return a generic error message so attackers cannot tell whether an email exists. Log and alert on spikes, check for breached passwords at signup, and offer two-factor authentication for sensitive accounts.
04
How do you protect certain routes in React so that only logged-in users can access them?
Create a ProtectedRoute or layout component that reads auth state from context. If the user is not authenticated, return <Navigate to='/login' replace state={{ from: location }} />, otherwise render the Outlet or children. Show a loader while the session check is pending to avoid flicker. Remember that this is only for user experience; real protection must happen on the Express API by verifying tokens on every request.
A closure is a function that remembers the variables of the scope where it was created, even after that outer function has returned. For example, a makeCounter function can return an inner function that increments a private count variable. Closures power data privacy, function factories, memoization, debounce and throttle helpers, and React hooks, where event handlers capture props and state from the render that created them.
06
How does the JavaScript event loop work with microtasks and macrotasks?
JavaScript runs one call stack. When the stack is empty, the event loop first drains the entire microtask queue, which holds Promise callbacks, queueMicrotask and await continuations. Only then does it pick the next macrotask, such as a setTimeout callback, an I/O callback or a UI event. This is why a resolved Promise callback always runs before a setTimeout of 0 ms scheduled at the same time.
07
How do you protect a public Express API from abuse and brute-force traffic?
Add rate limiting with express-rate-limit, using a Redis store when running multiple instances, and apply stricter limits to login and OTP routes. Set request body size limits, validate all input, require authentication on non-public routes, add helmet headers and timeouts. At the edge, a CDN or web application firewall can block bots and DDoS traffic before it reaches Node. Return 429 with a Retry-After header.
08
What is the difference between a shallow copy and a deep copy?
A shallow copy duplicates only the top level, so nested objects are still shared references. Spread syntax and Object.assign make shallow copies. A deep copy duplicates every nested level. structuredClone(obj) is the built-in way in modern browsers and Node.js and supports Dates, Maps and Sets, while JSON.parse(JSON.stringify(obj)) loses functions, undefined and Dates. In React state updates, shallow copies of each changed level are usually enough.
09
The React app works locally but shows CORS errors after deployment. What would you check?
Check that the production frontend origin, including https and exact subdomain, is in the API's allowed origins list, and that no trailing slash mismatch exists. If cookies are sent, the server needs credentials: true with a specific origin, not *, and the client needs credentials: 'include' or withCredentials. Also confirm the preflight OPTIONS request is not blocked by a proxy or auth middleware returning 401.
10
How should passwords be stored, and why use bcrypt?
Never store plain or reversibly encrypted passwords. Store a slow, salted hash. bcrypt generates a random salt per password, so identical passwords produce different hashes and rainbow tables fail, and its cost factor, typically 10 to 12, makes brute force expensive. At login use bcrypt.compare. Argon2id is a modern alternative. Fast hashes like MD5 or SHA-256 alone are unsuitable for passwords.
11
How does async/await improve on Promise chains, and how do you handle errors with it?
async/await is syntax over Promises that lets asynchronous code read top to bottom like synchronous code. An async function always returns a Promise, and await pauses that function until the Promise settles. Errors are handled with try/catch/finally instead of chained catch calls. A common mistake is awaiting independent calls one after another; use Promise.all to run them in parallel when they do not depend on each other.
12
How do you add a timeout to a fetch call in Node.js?
Node 18 and later ship a global fetch. Use AbortSignal.timeout to cancel slow requests.
async function getJSON(url) {
try {
const res = await fetch(url, { signal: AbortSignal.timeout(5000) });
if (!res.ok) throw new Error(`HTTP ${res.status}`);
return await res.json();
} catch (err) {
if (err.name === 'TimeoutError') console.error('Request timed out');
throw err;
}
}
Without a timeout, a hung upstream can hold connections open indefinitely.
13
How would you structure the folders of a production Express API?
A layered structure keeps responsibilities clear: src/routes for route definitions, src/controllers for request and response handling, src/services for business logic, src/models for Mongoose schemas, src/middlewares for auth, validation and errors, src/config for environment and database setup, src/utils for helpers, and tests alongside or in a tests folder. Keep app.js, which builds the app, separate from server.js, which listens, so tests can import the app.
React.memo wraps a component so it skips re-rendering when its props are shallowly equal to the previous props. It helps with expensive components that re-render often because the parent re-renders, such as rows in a large table. It fails if you pass new object, array or function props every render, so pair it with useMemo and useCallback. Measure with React DevTools Profiler before adding it.
16
Read a JSON file and write an updated version using fs/promises and async/await.
Situation: while testing an order history feature, I noticed changing the order id in the URL showed another user's order. Task: fix this insecure direct object reference before release. Action: I added an ownership check in the service layer so queries always filter by the logged-in user's id, wrote tests for cross-user access, and reviewed similar endpoints. Result: we fixed four endpoints and added ownership checks to our review checklist.
A fragment, written <>...</> or <Fragment key={id}>, groups children without adding an extra DOM element, which keeps tables and flex layouts valid. A portal, created with createPortal(child, domNode), renders children into a different DOM node such as document.body while keeping them in the same React tree, so context and event bubbling still work. Portals are used for modals, tooltips and toasts that must escape overflow hidden containers.
20
Your API becomes unresponsive whenever a user uploads an image for resizing. What is happening and how do you fix it?
Resizing is CPU-bound work running on the main thread, so it blocks the event loop and every other request waits. Fix it by moving the work off the main thread: use a worker_threads pool, a native library like sharp that works in the libuv pool, or better, push a job to a queue such as BullMQ with Redis and process it in a separate worker service, then notify the client.
21
How would you explain a technical decision, such as choosing REST over GraphQL, to a non-technical manager?
Situation: my manager asked why we were not using GraphQL like a competitor. Task: explain without jargon. Action: I compared it to ordering from a fixed menu versus a custom kitchen, explained that our screens need simple, predictable data, and that REST meant faster delivery with our team's current skills, lower risk and easier caching. Result: the manager approved the plan and we revisited the idea later.
22
You are building a registration form with ten fields and validation. How do you manage its state?
For a simple form, keep a single state object and update fields with a shared handler using the input name, for example setForm({ ...form, [e.target.name]: e.target.value }). Validate on blur and submit, storing errors in another object. For larger forms, use React Hook Form with a zod schema, which reduces re-renders and shares the validation schema with the Express backend.
23
A for loop using var with setTimeout inside prints 3, 3, 3 instead of 0, 1, 2. Why, and how do you fix it?
var is function-scoped, so all three callbacks close over the same variable, which has become 3 by the time the timers fire. The simplest fix is to declare the loop variable with let, because let creates a fresh binding for every iteration. Older fixes include wrapping the body in an immediately invoked function or passing i as the third argument to setTimeout.
24
Write a reusable validation middleware for Express using zod.
Situation: our order API started timing out every evening at peak. Task: as the on-call developer I had to restore it. Action: I checked logs and metrics, found event-loop lag spikes, and traced them to a synchronous CSV export running inside a request. I moved the export to a background job and added event-loop lag alerts. Result: p95 latency dropped from 4 seconds to 300 ms and timeouts stopped.
26
Write a React Testing Library test for a counter button.
Query by role and simulate real user events.
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import Counter from './Counter';
test('increments the count when + is clicked', async () => {
const user = userEvent.setup();
render(<Counter />);
expect(screen.getByText('Count: 0')).toBeInTheDocument();
await user.click(screen.getByRole('button', { name: '+' }));
expect(screen.getByText('Count: 1')).toBeInTheDocument();
});
toBeInTheDocument comes from @testing-library/jest-dom.
27
How do you debug a JavaScript bug that a user reports but you cannot reproduce locally?
Situation: a user reported a checkout button doing nothing, but it worked for me. Task: find the cause quickly. Action: I asked for browser, device and steps, checked our error-tracking logs, and found a TypeError only on an older Safari version due to an unsupported method. I reproduced it on a cloud device, added a polyfill and a test. Result: the fix shipped the same day and I added browser targets to our build.
28
What are the important changes in Express 5 compared to Express 4?
Express 5 requires a modern Node version and forwards rejected Promises from async handlers to error middleware. Route path syntax moved to a newer path-to-regexp, so wildcards must be named, like /*splat, and regex-style characters in strings are no longer supported. Deprecated methods like res.send(status, body) were removed, req.query is read-only, and req.body is undefined unless a body parser runs.
29
Your MERN app works on your laptop but crashes after deployment to a server. How do you troubleshoot?
Read the server logs first. Common causes are missing environment variables, a different Node version, the database host or IP allowlist blocking the server, hardcoded localhost URLs, wrong PORT handling, devDependencies missing in production, case-sensitive file names on Linux, and the frontend API base URL pointing to localhost. Reproduce with the same Node version or a Docker image, and validate config at startup.
30
What is NoSQL injection and how do you prevent it in a MongoDB app?
NoSQL injection happens when user input is placed directly into a query and contains operators. For example, a login body of { email: 'a@b.com', password: { $ne: null } } could match any password if passed straight to findOne. Prevent it by validating types with zod or Joi, casting inputs to strings, stripping keys starting with $ or containing dots, enabling Mongoose strictQuery, and never building $where queries from user input.
31
Users must upload files up to 200 MB. How would you handle this in a MERN application?
Avoid buffering big files in the API server's memory. The best approach is to have the backend generate a pre-signed URL for object storage such as S3, then let the React client upload directly to storage, with multipart upload for large files. If uploads must go through Express, use multer with disk storage or stream to storage, enforce size and type limits, and save only metadata in MongoDB.
32
Your Node.js API runs on an 8-core server but CPU monitoring shows only one core busy. What do you do?
A Node process runs JavaScript on a single thread, so one instance uses about one core. Run multiple instances: PM2 in cluster mode with -i max, the cluster module, or several containers behind a load balancer such as Nginx. Because instances do not share memory, move sessions, caches and rate-limit counters to Redis, and make sure the app is stateless so any instance can serve any request.
33
Your MERN product is expected to grow from 1,000 to 1 lakh daily users. How would you prepare the architecture?
Make the API stateless and run multiple instances behind a load balancer with autoscaling. Serve the React build and media from a CDN. Move sessions, rate limits and caches to Redis. Optimise MongoDB with proper indexes, a replica set, read preferences for analytics and plan sharding if data grows. Push slow work to queues, add monitoring and alerts, and load test before launch to find limits.
34
What is the difference between == and === in JavaScript?
== compares values after type coercion, so 0 == '0' and null == undefined are both true. === compares value and type without coercion, so 0 === '0' is false. Coercion rules are hard to remember and cause subtle bugs, so teams use === everywhere, often enforced by an ESLint rule. A common accepted exception is value == null, which checks for both null and undefined in one step.
helmet sets HTTP security headers in one line: Content-Security-Policy to restrict script sources, Strict-Transport-Security to force HTTPS, X-Content-Type-Options nosniff, X-Frame-Options or frame-ancestors to prevent clickjacking, and Referrer-Policy. It also removes the X-Powered-By header that reveals Express. It is not a complete security solution, but it is a cheap, standard first layer in every production API.
36
What is PM2 and why is it used to run Node.js in production?
PM2 is a process manager for Node.js. It keeps the app alive by restarting it after crashes, runs multiple instances in cluster mode to use all CPU cores, supports zero-downtime reloads with pm2 reload, manages logs and starts apps on server boot. Configuration lives in an ecosystem.config.js file. On container platforms like Kubernetes, the orchestrator often replaces PM2's restart and scaling role.
Which companies hire MERN Stack professionals in Jaipur?
Companies with large offices in Jaipur include Nagarro. Use their company pages below to practise employer-specific sets.
Are MERN Stack interviews in Jaipur different from other cities?
The questions depend more on the company than the city. Jaipur's mix of employers decides the style: Many candidates here also apply to remote and hybrid roles with Delhi NCR teams
How do I find MERN Stack jobs in Jaipur?
Shortlist companies with Jaipur offices, keep your CV focused on MERN Stack projects, apply on company career pages and job portals with the Jaipur filter, and practise the questions on this page before each interview.