Frontend Development Interview Questions in Mumbai
36 practice questions with answers for Frontend Development interviews in Mumbai.
Frontend Development jobs in Mumbai
Mumbai is India's financial capital, so banks, fintechs, insurers, media and consulting firms drive much of the hiring. Data and QA interviews often include domain questions on payments, risk, trading or customer analytics.
Below are 36 questions that match the mix of service, product and domain employers hiring frontend developers in Mumbai.
Solved for MumbaiStart practicing
3 QUESTIONS FREESign up to unlock the full question bank and track your progress.Unlock every question and answer, then continue practising company-wise and package-wise.
Throttle allows a function to run at most once in a given interval:
function throttle(fn, wait) {
let last = 0;
return function (...args) {
const now = Date.now();
if (now - last >= wait) {
last = now;
fn.apply(this, args);
}
};
}
window.addEventListener('scroll', throttle(updateProgressBar, 100));
Unlike debounce, which waits for calls to stop, throttle fires regularly during continuous events such as scrolling or dragging.
02
Collect Core Web Vitals from real users with the web-vitals library.
Report metrics to your analytics endpoint:
import { onLCP, onINP, onCLS } from 'web-vitals';
function send(metric) {
const body = JSON.stringify({
name: metric.name,
value: metric.value,
rating: metric.rating,
page: location.pathname
});
navigator.sendBeacon('/analytics/vitals', body);
}
onLCP(send);
onINP(send);
onCLS(send);
sendBeacon delivers data reliably even when the page is closing. Aggregate at the 75th percentile per page.
Load flags once and check them where needed:
let flags = {};
export async function loadFlags(userId) {
const res = await fetch('/api/flags?user=' + encodeURIComponent(userId));
flags = res.ok ? await res.json() : {};
}
export const isEnabled = (name) => flags[name] === true;
// usage
await loadFlags(currentUser.id);
if (isEnabled('new-checkout')) renderNewCheckout();
else renderOldCheckout();
Defaulting to off when the request fails keeps the stable path. Flags enable gradual rollouts, but old flags must be removed to avoid clutter.
04
Write a Vitest unit test for an INR price formatter.
Tokens go in CSS with the @theme directive:
@import 'tailwindcss';
@theme {
--color-brand-500: #2563eb;
--color-brand-700: #1d4ed8;
--font-display: 'Poppins', sans-serif;
--radius-card: 12px;
}
/* usage in markup */
/* <button class='bg-brand-500 hover:bg-brand-700 rounded-card font-display'> */
Tailwind generates utilities like bg-brand-500 from these variables and also exposes them as CSS custom properties for use in plain CSS.
06
What is prefers-reduced-motion and why should you respect it?
prefers-reduced-motion is a media query that detects when users have asked their operating system to minimise animation, often because motion causes dizziness or nausea. Inside @media (prefers-reduced-motion: reduce), you should remove or tone down parallax, large transitions and auto-playing animations, while keeping essential feedback such as simple fades. It is a small change that makes interfaces accessible to people with vestibular disorders.
07
Create a new Vite project with TypeScript and run it.
Use the official scaffolding tool:
npm create vite@latest my-app -- --template vanilla-ts
cd my-app
npm install
npm run dev # starts the dev server with HMR
npm run build # type-checks and builds to dist/
npm run preview # serves the production build locally
Templates also exist for react-ts, vue-ts and svelte-ts. The dist folder contains static files ready for any web server or CDN.
08
What is the difference between :is() and :where()?
Both take a list of selectors and match any of them, reducing repetition, for example :is(h1, h2, h3) a. The difference is specificity: :is() takes the specificity of its most specific argument, while :where() always has zero specificity. That makes :where() perfect for resets and library defaults that users should easily override, while :is() keeps normal priority.
The worker runs on its own thread and talks through messages:
// worker.js
self.onmessage = (e) => {
const total = e.data.reduce((sum, row) => sum + row.amount, 0);
self.postMessage(total);
};
// main.js
const worker = new Worker(new URL('./worker.js', import.meta.url), { type: 'module' });
worker.postMessage(bigArrayOfRows);
worker.onmessage = (e) => showTotal(e.data);
The UI stays responsive. Data is copied between threads, so for huge binary data use transferable objects like ArrayBuffer.
10
Test a DOM form with Testing Library and user-event.
Simulate real typing and clicks:
import { screen } from '@testing-library/dom';
import userEvent from '@testing-library/user-event';
import { mountSignupForm } from './signup';
it('shows an error for an invalid mobile number', async () => {
document.body.innerHTML = '<div id=app></div>';
mountSignupForm(document.getElementById('app'));
const user = userEvent.setup();
await user.type(screen.getByLabelText('Mobile'), '12345');
await user.click(screen.getByRole('button', { name: 'Sign up' }));
expect(screen.getByRole('alert')).toHaveTextContent('Enter a valid mobile number');
});
toHaveTextContent comes from @testing-library/jest-dom.
Extend HTMLElement and register a tag name containing a hyphen:
class RatingStars extends HTMLElement {
connectedCallback() {
const value = Number(this.getAttribute('value') || 0);
const root = this.attachShadow({ mode: 'open' });
root.innerHTML = `<style>span { color: var(--star-color, orange); }</style>
<span role='img' aria-label='${value} out of 5'>${'*'.repeat(value)}</span>`;
}
}
customElements.define('rating-stars', RatingStars);
// usage: <rating-stars value='4'></rating-stars>
The custom property lets the page theme it despite Shadow DOM isolation.
12
What is the guiding principle of Testing Library and why query by role?
Testing Library encourages testing the way users interact: find elements by what they see and hear, not by implementation details like class names or component internals. getByRole('button', { name: 'Submit' }) is preferred because it also verifies the element is accessible. Tests written this way survive refactors and catch real accessibility problems. Test ids are a last resort when no accessible query works.
13
Tell me about a time you convinced your team to adopt a new tool like TypeScript.
Situation: our JavaScript app kept breaking after API changes. Task: I believed TypeScript would help but the team worried about slowing down. Action: I converted one small module as a pilot, showed three bugs it caught, and proposed gradual adoption with no deadline pressure. Result: the team agreed, we migrated over two quarters, and production errors related to wrong data shapes dropped significantly.
14
How do HTTP/2 and HTTP/3 change frontend bundling strategy?
HTTP/1.1 allowed few parallel requests per domain, so developers concatenated everything into one big bundle and used domain sharding. HTTP/2 multiplexes many requests over one connection, and HTTP/3 runs over QUIC to avoid head-of-line blocking on lossy mobile networks. This makes moderate chunk splitting efficient and improves caching, because changing one module invalidates only its chunk. Still, too many tiny files add overhead.
15
At a high level, how does React's virtual DOM and reconciliation work?
React components return a description of the UI as a tree of elements. When state changes, React renders a new tree and compares it with the previous one, a process called reconciliation, then applies only the minimal DOM changes needed. Keys help it match list items correctly. The React Compiler now adds automatic memoisation, reducing unnecessary re-renders without manual useMemo calls.
16
What is the difference between a Web Worker and a Service Worker?
A Web Worker runs JavaScript on a background thread for a single page, used for heavy computations like parsing files or image processing without freezing the UI. A Service Worker sits between the page and the network for an entire origin, intercepting requests to enable offline support, caching and push notifications. It has a lifecycle of install, activate and fetch, and it works only over HTTPS.
17
How would you explain a difficult JavaScript concept to a junior teammate?
Situation: a junior developer kept writing bugs with async code. Task: I wanted them to understand promises rather than copy fixes. Action: I drew the event loop on a whiteboard, wrote three small console examples for them to predict, and then reviewed one of their real pull requests together. Result: within two weeks, their async bugs stopped and they started explaining the concept to the next new joiner.
18
Compare CSS Modules, CSS-in-JS and utility-first CSS.
CSS Modules scope class names per file at build time, giving plain CSS with no runtime cost. CSS-in-JS libraries write styles in JavaScript with dynamic props, but runtime versions add bundle size and slow server rendering, so zero-runtime options are now preferred. Utility-first frameworks like Tailwind compose small classes in markup, producing small CSS and consistent spacing. The choice depends on team preference and rendering strategy.
19
What is the same-origin policy and how does postMessage work around it?
An origin is the combination of scheme, host and port. The same-origin policy stops scripts from reading data or DOM of another origin, such as an embedded iframe from a payment provider. window.postMessage lets two windows or iframes exchange messages safely across origins. The receiver must check event.origin against an allow-list, and the sender should specify the exact target origin instead of '*'.
20
How do you work with QA engineers as a frontend developer?
I treat QA as partners rather than gatekeepers. I share feature details and edge cases early, give them testable builds on staging, and add stable test ids or accessible labels that help automation. When bugs come in, I ask for clear steps and environment details instead of arguing. In my last project, this collaboration cut bug reopen rates because we agreed on acceptance criteria before development.
Cache-first serves from cache and falls back to network, ideal for versioned static assets. Network-first tries the network and falls back to cache, suited to HTML and API data that should be fresh. Stale-while-revalidate returns the cached copy immediately while fetching an update in the background, good for avatars or non-critical data. Libraries like Workbox implement these strategies and handle cache expiry and versioning.
preconnect opens an early connection, including DNS, TCP and TLS, to an important third-party origin like a font or API server. preload fetches a resource needed for the current page at high priority, such as the hero image or main font. prefetch downloads resources likely needed for the next navigation at low priority during idle time. Overusing preload wastes bandwidth and can delay more critical resources.
24
What is the difference between lab data and field data in performance?
Lab data comes from controlled tests like Lighthouse or WebPageTest on a fixed device and network, which is great for debugging and comparing changes. Field data, or real user monitoring, comes from actual visitors through the Chrome UX Report or your own analytics, capturing real devices, networks and behaviour. Google ranks on field data. A page can score well in the lab but fail in the field on low-end phones.
25
Explain the utility types Partial, Pick, Omit and Record.
Partial<T> makes all properties optional, useful for update payloads. Pick<T, 'id' | 'name'> creates a type with only selected keys, and Omit<T, 'password'> removes keys, handy for public user objects. Record<K, V> builds an object type with keys K and values V, such as Record<'en' | 'hi', string> for translations. Others include Required, Readonly, ReturnType and Awaited.
26
Which tsconfig options are important for a strict codebase?
strict: true enables a family of checks, including strictNullChecks and noImplicitAny, and should be on for new projects. noUncheckedIndexedAccess adds undefined to array and record lookups, catching many runtime errors. exactOptionalPropertyTypes, noImplicitOverride and noFallthroughCasesInSwitch add more safety. For bundler-based apps, moduleResolution: bundler, isolatedModules and verbatimModuleSyntax keep TypeScript aligned with how tools like Vite compile code.
Design tokens are named values for design decisions, such as color-primary, space-4 or radius-md, stored in one place and shared by designers and developers. They are often kept in JSON and converted into CSS custom properties, Tailwind theme values or mobile platform formats. Changing a token updates every component consistently, which makes rebranding or dark mode much easier than hunting hard-coded values.
28
QA reports a bug that happens only in Safari on iPhone. How do you debug it?
I would reproduce it on a real iPhone or simulator and connect Safari Web Inspector from a Mac to see console errors and network requests. Common causes are unsupported APIs, date parsing differences, 100vh and viewport issues, and input zoom on small fonts. Cloud device platforms help if no device is available. After fixing it, I would add WebKit to the Playwright browser matrix to catch similar issues.
29
What is the testing pyramid, and how does the testing trophy differ?
The testing pyramid suggests many fast unit tests, fewer integration tests and very few slow end-to-end tests. The testing trophy, popular for frontend work, puts most effort into integration tests that render components with real behaviour, supported by static analysis like TypeScript and ESLint at the base. Both aim for confidence at reasonable cost; the trophy reflects that UI bugs often appear where units connect.
30
Configure cache headers for hashed assets and HTML in Nginx.
Long cache for fingerprinted files, revalidation for HTML:
location ~* \.(js|css|woff2|avif|webp|svg)$ {
add_header Cache-Control 'public, max-age=31536000, immutable';
}
location / {
try_files $uri /index.html;
add_header Cache-Control 'no-cache';
}
This works only when build tools add content hashes to filenames. Returning users then load assets from cache instantly, while every deploy still reaches them through fresh HTML.
31
What is a JavaScript bundler and why do frontend projects need one?
A bundler takes your source files, follows imports to build a dependency graph and outputs optimised files for the browser. Along the way it transpiles TypeScript and JSX, processes CSS, handles images, minifies code, removes unused exports and adds content hashes for caching. Native ES modules work in browsers, but loading hundreds of unoptimised files is slow, so production builds still use bundlers like Vite, webpack or Rspack.
32
You must change a complex legacy module that has no tests. How do you proceed safely?
I would first write characterisation tests that capture the current behaviour, including odd cases, by calling the module with real inputs and recording outputs. With that safety net, I make small refactors to separate logic from DOM or network code, adding focused tests as I go. Then I implement the change. Small commits and feature flags let me release gradually and roll back quickly if something breaks.
33
How do environment variables work in frontend builds, and what is the risk?
Build tools replace variables at build time, for example Vite exposes only variables prefixed with VITE_ through import.meta.env. The values become plain text inside the JavaScript bundle that anyone can read. So frontend environment variables must never hold secrets like private API keys or database passwords; those belong on a server or backend proxy. Use them only for public configuration such as API base URLs.
34
npm audit reports a critical vulnerability in a dependency. What do you do?
I would check whether the vulnerable package is actually used in the production bundle or only in build tooling, and whether the vulnerable code path is reachable. Then I would update to a patched version, using npm update or overrides for transitive dependencies. If no fix exists, I would look for an alternative package or mitigate. Automated tools like Dependabot or Renovate keep dependencies current.
35
Tell me about a time you pushed back on releasing code without tests.
Situation: near a release deadline, a teammate suggested skipping tests for a new checkout coupon feature. Task: I felt the risk to payments was too high. Action: I proposed covering only the critical paths, valid, expired and invalid coupons, and wrote those tests myself in two hours. Result: one test caught a discount rounding bug before release, and the feature shipped on time.
36
How do HTTP caching headers work for frontend assets?
Cache-Control controls caching. Files with content hashes in their names, like app.3f9a2c.js, can use max-age=31536000, immutable because a new build produces a new filename. HTML should use no-cache, which forces revalidation so users get new asset references after a deploy. ETag and Last-Modified allow cheap revalidation with 304 Not Modified responses. Wrong headers cause either stale apps or needless re-downloads.
Which companies hire Frontend Development professionals in Mumbai?
Companies with large offices in Mumbai include Razorpay, Paytm, Google, BrowserStack. Use their company pages below to practise employer-specific sets.
Are Frontend Development interviews in Mumbai different from other cities?
The questions depend more on the company than the city. Mumbai's mix of employers decides the style: Data and QA interviews often include domain questions on payments, risk, trading or customer analytics
How do I find Frontend Development jobs in Mumbai?
Shortlist companies with Mumbai offices, keep your CV focused on Frontend Development projects, apply on company career pages and job portals with the Mumbai filter, and practise the questions on this page before each interview.