36 practice questions with answers for MERN Stack interviews in Indore.
MERN Stack jobs in Indore
Indore is one of central India's fastest-growing IT hubs, with IT-services campuses and product start-ups in its Super Corridor and IT parks. Companies here hire many freshers and value hands-on project work.
Below are 36 questions that match the mix of service, product and domain employers hiring mern stack developers in Indore.
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01
Compare Promise.all, Promise.allSettled, Promise.race and Promise.any.
Promise.all resolves with all results when every Promise fulfils and rejects as soon as any one rejects. Promise.allSettled always waits for all and returns status objects, useful when partial failure is fine. Promise.race settles with the first Promise to settle, often used for timeouts. Promise.any resolves with the first fulfilment and rejects with an AggregateError only if all reject, useful for trying multiple mirrors.
02
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.
03
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.
04
How do you implement code splitting in a React app?
Use dynamic import with React.lazy, for example const Admin = lazy(() => import('./Admin')), and wrap it in a Suspense boundary with a fallback. Bundlers like Vite then create a separate chunk that loads only when needed. Split by route first, then by heavy components like charts, editors and maps. Preload likely next routes on hover, and check results with a bundle analyser such as rollup-plugin-visualizer.
05
What is a custom hook and what rules must hooks follow?
A custom hook is a function whose name starts with use and that calls other hooks to package reusable stateful logic, such as useFetch, useDebounce or useAuth. Each component using it gets its own state. Hooks must be called at the top level of a component or another hook, never inside conditions, loops or nested functions, so React can match state by call order. The eslint-plugin-react-hooks rules enforce this.
Debounce delays a call until a quiet period has passed. Each new call clears the previous timer.
function debounce(fn, delay = 300) {
let timer;
return function (...args) {
clearTimeout(timer);
timer = setTimeout(() => fn.apply(this, args), delay);
};
}
const onSearch = debounce((q) => fetchResults(q), 400);
It relies on a closure over timer and keeps this and arguments intact.
07
How do you implement graceful shutdown for a Node.js API during deployments?
Listen for SIGTERM and SIGINT. When received, stop accepting new connections with server.close(), let in-flight requests finish, close database connections with mongoose.connection.close() and other resources, then exit with code 0. Set a timeout, for example 10 seconds, after which you force exit. In Kubernetes or behind a load balancer, also fail the readiness check first so traffic stops reaching the instance.
08
Show the use of $push, $addToSet, $inc and $pull update operators.
REST models data as resources identified by URLs, uses standard HTTP methods, is stateless, and returns representations like JSON with proper status codes. Use nouns, for example GET /orders and POST /orders. GET, PUT, DELETE, HEAD and OPTIONS are idempotent: repeating them gives the same server state. POST is not idempotent, and PATCH usually is not, which matters when clients retry requests.
10
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.
11
A developer accidentally pushed the .env file with the MongoDB password and JWT secret to a public repository. What do you do?
Treat the secrets as compromised immediately. Rotate the MongoDB user password, the JWT secret and any API keys; rotating the JWT secret logs everyone out, which is acceptable. Check database and cloud logs for suspicious access. Remove the file, add .env to .gitignore, and purge it from history with a tool like git filter-repo, though rotation matters more. Enable secret scanning and pre-commit hooks to prevent repeats.
12
What is the difference between useMemo and useCallback?
useMemo caches the result of a calculation, recomputing only when dependencies change, for example a filtered and sorted list. useCallback caches a function reference, so useCallback(fn, deps) equals useMemo(() => fn, deps). They help when passing values or callbacks to memoized children or as effect dependencies. They have their own cost, so do not wrap everything; the React Compiler can now add this memoization automatically.
13
Tell me about a time you had to deliver a frontend feature under a tight deadline.
Situation: a client demo was moved two days earlier and our filters page was half done. Task: deliver a working version in time. Action: I agreed a reduced scope with the lead, reused existing components, built the three most-used filters first and wrote quick tests for them, and kept the team updated each evening. Result: the demo went well and the remaining filters shipped in the next sprint.
14
Design a payment endpoint so that client retries never charge a user twice.
Make the endpoint idempotent with an idempotency key. The client generates a unique key per checkout and sends it in a header. The server stores the key with a unique index along with the request hash and the result. If the same key arrives again, return the stored response instead of reprocessing. Use a status field for in-progress requests and verify final status through the payment gateway webhook.
15
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.
16
Duplicate user accounts with the same email are being created even though the code checks for an existing email first. Why, and how do you fix it?
Check-then-insert is a race condition: two concurrent requests both see no user and both insert. The real fix is a unique index on email, normalised to lowercase, so the database rejects the second insert with error code 11000. Catch that error and return 409 Conflict. Clean up existing duplicates before building the index, otherwise index creation fails.
17
Write a GitHub Actions workflow that tests and builds a MERN project on every pull request.
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 ${res.status}`);
return await res.json();
} catch (err) {
if (err.name === 'TimeoutError') console.error('Request timed out');
throw err;
}
}
Without a timeout, a hung upstream can hold connections open indefinitely.
19
What is PM2 and why is it used to run Node.js in production?
PM2 is a process manager for Node.js. It keeps the app alive by restarting it after crashes, runs multiple instances in cluster mode to use all CPU cores, supports zero-downtime reloads with pm2 reload, manages logs and starts apps on server boot. Configuration lives in an ecosystem.config.js file. On container platforms like Kubernetes, the orchestrator often replaces PM2's restart and scaling role.
20
Write a docker-compose file to run a MERN app locally.
Three services on one network:
services:
mongo:
image: mongo:8
volumes:
- mongo-data:/data/db
api:
build: ./server
environment:
MONGO_URI: mongodb://mongo:27017/shop
JWT_SECRET: dev-secret
ports:
- "5000:5000"
depends_on:
- mongo
web:
build: ./client
ports:
- "8080:80"
depends_on:
- api
volumes:
mongo-data:
The API reaches the database through the service name mongo.
21
What are Mongoose middleware hooks and give a common use case.
Mongoose middleware are functions that run before or after operations such as save, validate, deleteOne, findOneAndUpdate and aggregate. A classic pre-save hook hashes a password only if it was modified: if (!this.isModified('password')) return; this.password = await bcrypt.hash(this.password, 12). Post hooks are used for logging or syncing. Note that query middleware like findOneAndUpdate does not trigger save hooks.
22
Write Express middleware that logs method, URL, status code and response time.
Hook into the response finish event to know the final status.
function requestLogger(req, res, next) {
const start = process.hrtime.bigint();
res.on('finish', () => {
const ms = Number(process.hrtime.bigint() - start) / 1e6;
console.log(`${req.method} ${req.originalUrl} ${res.statusCode} ${ms.toFixed(1)}ms`);
});
next();
}
app.use(requestLogger);
In production, use a structured logger like pino instead of console.log.
23
Design a secure forgot-password flow for a MERN application.
The user submits an email; always respond with the same message whether or not it exists. Generate a random token with crypto.randomBytes, store only its hash with an expiry of around 15 minutes, and email a link with the raw token. On submission, hash the token, find a matching unexpired record, set the new hashed password, delete the token and invalidate existing sessions. Rate limit the endpoint.
24
Your app needs user, editor and admin roles with different permissions. How would you implement it?
Store a role, or a list of permissions, on the user document and include it in the token or load it in the auth middleware. Write an authorize middleware that checks allowed roles or permissions per route, for example authorize('admin'). Add ownership checks in services, such as an editor editing only their own articles. In React, hide buttons by role for user experience, but always enforce rules on the server.
25
What is the Context API and when should you avoid it?
Context passes data through the component tree without prop drilling. You create it with createContext, provide a value with a Provider and read it with useContext, or the newer use API. It suits low-frequency global data like theme, language and logged-in user. Avoid it for frequently changing large state, because every consumer re-renders when the value changes; split contexts or use Zustand or Redux Toolkit instead.
26
How does routing work in a React single-page app using React Router?
React Router maps URL paths to components without full page reloads. You define routes, often with createBrowserRouter or Routes and Route elements, including nested routes that render inside a parent layout through Outlet. Link and NavLink change the URL, useNavigate navigates in code, and useParams reads dynamic segments like /products/:id. Recent versions also provide loaders and actions for data fetching and form submission.
27
Tell me about a deployment that went wrong and what you learned.
Situation: a release broke login for all users because a new environment variable was missing in production. Task: restore service fast. Action: I rolled back to the previous version within ten minutes, added the variable, and redeployed after testing on staging. Afterwards I added startup config validation and a post-deploy smoke test for login in our pipeline. Result: no config-related outage has happened since.
28
Group an array of order objects by their status using reduce.
reduce builds an object whose keys are statuses and values are arrays.
const grouped = orders.reduce((acc, order) => {
(acc[order.status] ||= []).push(order);
return acc;
}, {});
// { paid: [...], pending: [...] }
Newer runtimes also offer Object.groupBy(orders, (o) => o.status), which returns the same shape with a null prototype.
29
Walk me through a MERN project you built, end to end.
Situation: I built a job portal as my major project. Task: let recruiters post jobs and students apply with resumes. Action: I designed MongoDB collections for users, jobs and applications with indexes, built an Express REST API with JWT auth and role checks, a React frontend with React Router and TanStack Query, uploaded resumes to cloud storage, wrote API tests and deployed with Docker. Result: 60 classmates used it during placement season.
30
When should you embed documents and when should you reference them in MongoDB?
Embed when data is read together, belongs to one parent, and is bounded in size, such as addresses inside a user or line items inside an order. Reference with ObjectIds when the related data is large, grows without limit, is shared by many documents or is updated independently, such as products referenced by many orders. The rule is to design for your application's query patterns, not for normalisation.
31
What is the aggregation pipeline and what are its common stages?
The aggregation pipeline processes documents through a sequence of stages, each transforming the stream. Common stages are $match to filter, $group to aggregate with operators like $sum and $avg, $project or $addFields to shape fields, $sort, $limit, $skip, $lookup to join another collection, $unwind to flatten arrays and $facet for multiple outputs. Put $match and $sort early so they can use indexes and reduce the data volume.
32
Write a function to flatten a nested array without using Array.prototype.flat.
Recursion with reduce is the cleanest approach.
function flatten(arr) {
return arr.reduce((acc, item) =>
Array.isArray(item) ? acc.concat(flatten(item)) : acc.concat(item), []);
}
flatten([1, [2, [3, [4]]], 5]); // [1, 2, 3, 4, 5]
For very deep arrays, an iterative version with an explicit stack avoids hitting the recursion limit.
33
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.
34
What does a CI/CD pipeline for a MERN application usually include?
On every pull request, continuous integration installs dependencies with npm ci, runs lint, type checks, unit and integration tests, and builds the frontend. On merge to main, continuous delivery builds Docker images or artifacts, tags them with the commit, pushes to a registry, deploys to staging, runs smoke tests and then promotes to production, often with manual approval. GitHub Actions, GitLab CI and Jenkins are common tools.
35
Write a deep clone function that handles nested objects, arrays and dates.
In production prefer structuredClone, but interviewers often ask for a manual version.
function deepClone(value, seen = new WeakMap()) {
if (value === null || typeof value !== 'object') return value;
if (value instanceof Date) return new Date(value);
if (seen.has(value)) return seen.get(value);
const copy = Array.isArray(value) ? [] : {};
seen.set(value, copy);
for (const key of Object.keys(value)) copy[key] = deepClone(value[key], seen);
return copy;
}
The WeakMap prevents infinite loops on circular references.
36
Write a production Dockerfile for a Node.js Express API.
Keep the image small and run as a non-root user.
FROM node:22-alpine
WORKDIR /app
ENV NODE_ENV=production
COPY package*.json ./
RUN npm ci --omit=dev
COPY src ./src
USER node
EXPOSE 5000
CMD ["node", "src/server.js"]
Copying package files first lets Docker cache the dependency layer, so code changes rebuild quickly. Add a .dockerignore with node_modules and .env.
Few big employers listed in Indore?
Many Indore candidates also apply to teams in Pune and to remote or hybrid roles. Practise the Pune set as well.
Which companies hire MERN Stack professionals in Indore?
Companies with large offices in Indore include many IT services and product firms. Use their company pages below to practise employer-specific sets.
Are MERN Stack interviews in Indore different from other cities?
The questions depend more on the company than the city. Indore's mix of employers decides the style: Companies here hire many freshers and value hands-on project work
How do I find MERN Stack jobs in Indore?
Shortlist companies with Indore offices, keep your CV focused on MERN Stack projects, apply on company career pages and job portals with the Indore filter, and practise the questions on this page before each interview.