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MERN Stack Mock Interview: 20 Questions in 50 Minutes

Four rounds that copy a real interview. Answer aloud, then check the model answers.

Mock interview format

  1. Warm-up · 5 minutes — 4 questions
  2. Technical · 20 minutes — 7 questions
  3. Scenario · 15 minutes — 5 questions
  4. HR & behavioural · 10 minutes — 4 questions

Tip: record yourself and listen back. Most people talk too fast and skip the key idea.

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01

What are Jest and Vitest, and what is the philosophy of React Testing Library?

Warm-upTesting, Deployment and DevOpsEasy
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Jest and Vitest are JavaScript test runners with assertions, mocking and coverage; Vitest is Vite-native, very fast and Jest-compatible, so it is common in new React projects. React Testing Library renders components and encourages testing them the way users use them: find elements by role, label or text, fire user events, and assert on visible output, instead of testing internal state or implementation details.
02

Which HTTP status codes should a REST API commonly return?

Warm-upExpress.js and REST APIsEasy
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Use 200 OK for successful reads and updates, 201 Created after a POST that creates a resource, and 204 No Content for successful deletes. Use 400 for invalid input, 401 when authentication is missing or invalid, 403 when the user is authenticated but not allowed, 404 for missing resources, 409 for conflicts such as duplicate email, 422 for validation errors, 429 for rate limits and 500 for unexpected server errors.
03

What is the difference between props and state in React?

Warm-upReact FundamentalsEasy
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Props are inputs passed from a parent to a child component; they are read-only inside the child. State is data owned and managed by a component itself, created with useState or useReducer, and changing it triggers a re-render. A common pattern is that a parent keeps state and passes both the value and a setter function down as props to children.
04

What is Node.js and how does it handle many requests on a single thread?

Warm-upNode.js CoreEasy
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Node.js is a JavaScript runtime built on the V8 engine and libuv. Your JavaScript runs on one main thread, but I/O such as network calls, file reads and database queries is non-blocking. Node hands that work to the operating system or the libuv thread pool and continues with other requests; callbacks run when results are ready. This makes it efficient for I/O-heavy APIs, but weak for CPU-heavy work.
05

Write Express middleware that logs method, URL, status code and response time.

TechnicalExpress.js and REST APIsMedium
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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.
06

What are the important changes in Express 5 compared to Express 4?

TechnicalExpress.js and REST APIsMedium
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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.
07

What is the Context API and when should you avoid it?

TechnicalReact Hooks, State and PerformanceMedium
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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.
08

Set a refresh token in a secure httpOnly cookie after login.

TechnicalAuthentication and SecurityMedium
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Cookie flags carry most of the security. const refreshToken = jwt.sign({ sub: user.id }, process.env.REFRESH_SECRET, { expiresIn: '7d' }); const accessToken = jwt.sign({ sub: user.id, role: user.role }, process.env.JWT_SECRET, { expiresIn: '15m' }); res.cookie('refreshToken', refreshToken, { httpOnly: true, secure: true, sameSite: 'strict', path: '/api/auth/refresh', maxAge: 7 * 24 * 60 * 60 * 1000 }); res.json({ accessToken }); Read it later with the cookie-parser middleware via req.cookies.refreshToken.
09

What is a custom hook and what rules must hooks follow?

TechnicalReact Hooks, State and PerformanceMedium
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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.
10

How does routing work in a React single-page app using React Router?

TechnicalReact FundamentalsMedium
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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.
11

Show the use of $push, $addToSet, $inc and $pull update operators.

TechnicalMongoDB and MongooseMedium
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Update operators change parts of a document atomically: db.posts.updateOne({ _id: id }, { $inc: { views: 1 } }) db.posts.updateOne({ _id: id }, { $addToSet: { tags: 'mongodb' } }) db.posts.updateOne({ _id: id }, { $push: { comments: { text: 'Nice', at: new Date() } } }) db.posts.updateOne({ _id: id }, { $pull: { tags: 'old' } }) $addToSet avoids duplicates, while $push always appends. Add $slice inside $push to cap an array's length.
12

How do you implement graceful shutdown for a Node.js API during deployments?

ScenarioNode.js CoreMedium
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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.
13

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?

ScenarioMongoDB and MongooseMedium
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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.
14

Your Node.js API runs on an 8-core server but CPU monitoring shows only one core busy. What do you do?

ScenarioNode.js CoreMedium
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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.
15

Fast typing in a search box sometimes shows results for an older query. What causes this and how do you fix it?

ScenarioReact Hooks, State and PerformanceHard
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It is a race condition: responses arrive out of order, so a slow earlier request overwrites results of a newer one. In the effect, use an AbortController and abort the previous request in the cleanup, or keep an ignore flag set to true during cleanup and skip setState when it is set. Debouncing reduces requests, and TanStack Query handles this automatically by keying results to the query.
16

Your React production bundle is 3 MB and the first load is slow on mobile. What steps do you take?

ScenarioReact Hooks, State and PerformanceHard
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Analyse the bundle to find the largest modules. Split code by route with React.lazy, lazy load heavy libraries like charts and editors, replace large dependencies with lighter ones, import only needed functions so tree shaking works, and remove unused polyfills. Compress images, serve assets from a CDN with Brotli and long cache headers, and consider SSR or static rendering for landing pages. Track Core Web Vitals after each change.
17

How do you debug a JavaScript bug that a user reports but you cannot reproduce locally?

HR & behaviouralModern JavaScript (ES6+)Medium
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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.
18

Tell me about a time you improved the performance of a slow database-driven feature.

HR & behaviouralMongoDB and MongooseMedium
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Situation: our admin orders page took 8 seconds to load. Task: I was asked to fix it within a sprint. Action: I profiled the API, found a populate on every row plus an unindexed sort, added a compound index, replaced populate with a single $lookup projecting only needed fields, and used lean. Result: load time dropped below 600 ms and I shared an indexing checklist with the team.
19

Tell me about a time you received critical feedback on your UI code and how you responded.

HR & behaviouralReact FundamentalsEasy
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Situation: in my internship, a senior reviewer rejected my pull request because my components were large and repeated logic. Task: fix it without missing the sprint deadline. Action: I asked for examples, split the page into smaller components, extracted a custom hook for the shared fetching logic and added prop types. Result: the PR was approved the next day, and the reviewer used my hook in another feature.
20

Walk me through a MERN project you built, end to end.

HR & behaviouralTesting, Deployment and DevOpsMedium
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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.

Frequently asked questions

How do I use this mock interview?

Set a 50-minute timer, answer every question aloud (or ask a friend to read them), and open answers only after each round. Note every question where your answer missed the key idea.

How often should I take a mock?

Once a week while preparing and once two or three days before a real interview. The question order is fixed, so for fresh practice use the company and experience pages too.

More ways to practise

Company-wise MERN Stack questionsExperience-wise MERN Stack questionsRole-wise MERN Stack questionsCity-wise MERN Stack questionsMost asked MERN Stack questionsScenario-based questionsPractical / hands-on questionsHR & behavioural questionsOnline assessment questionsTechnical round questionsManagerial round questionsMERN Stack topic sheets