Frontend Development Interview Questions for 3 Years Experience
40 questions with answers for candidates with 3 years of experience — 4 easy, 34 medium, 2 hard.
What interviewers expect: 3 years experience
Three-year candidates are expected to own features and influence code quality. Expect frontend system design questions like an autocomplete or dashboard, advanced TypeScript, caching strategies, security practices and CI setup. Interviewers also probe code review habits, mentoring juniors and how you work with designers and backend teams.
This set is weighted towards TypeScript (7), Modern JavaScript (6), Browser APIs (5), CSS Architecture and Accessibility (5).
Frontend system design Advanced TypeScript Security practices Code review and mentoring
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01
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.
02
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.
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.
04
Make an embedded video keep a 16:9 ratio at any width.
Coverage shows which lines ran, not whether the behaviour was verified, so tests can execute code without meaningful assertions. I would explain this with an example and propose a balanced target, perhaps 80 percent on business logic, combined with quality measures like mutation testing, critical user flows covered by end-to-end tests and tracking escaped defects. Effort is better spent on risky areas than on trivial getters.
06
How can you communicate between multiple tabs of the same site?
BroadcastChannel lets same-origin tabs, windows and workers publish and subscribe to messages on a named channel, which suits syncing logout, theme or cart updates. The storage event fires in other tabs when localStorage changes, which is a simple fallback. SharedWorker can hold shared state for all tabs. Each option works only within the same origin, so cross-domain sync needs a server or postMessage.
07
The bundle analyser shows two copies of the same library. How do you fix it?
Duplicates happen when different packages depend on incompatible versions, so the package manager installs both. I would run npm ls library-name or pnpm why to see who requires which version, then update dependents to a common version, use overrides or resolutions to force one compatible version, or configure resolve.dedupe in Vite. Duplicate framework copies are especially harmful because they can break shared state and context.
08
Tell me about a time you received critical feedback in a code review.
Situation: a senior engineer rejected my pull request for a new dashboard because the CSS was hard to maintain. Task: I had to rework it without missing the sprint. Action: I asked for a short call to understand the concerns, refactored styles into reusable components with tokens, and added notes explaining my choices. Result: the PR was approved, and the components were later reused in two other screens.
09
Lighthouse shows a score of 95, but Search Console reports failing Core Web Vitals. Why?
Lighthouse is a single lab run on a simulated device, while Search Console uses field data from real Chrome users at the 75th percentile over 28 days. Real users may have slower phones, interact with the page, which affects INP and CLS later in the session, or see different content like ads and personalised blocks. I would add real user monitoring with attribution to find which pages and elements fail.
10
A new browser API you used does not work in older Safari versions. How do you handle it?
I would use feature detection, such as if ('share' in navigator), rather than browser sniffing, and provide a fallback like a copy-link button. Checking Can I Use and the project's browser support list before building avoids surprises. Where reasonable, a small polyfill can fill gaps. This progressive enhancement approach ensures the core feature works everywhere while newer browsers get a better experience.
Tree shaking removes exports that are never imported from the final bundle. It relies on static ES module imports and exports. It fails with CommonJS modules, dynamic property access on imported objects, and code with side effects that the bundler must keep. Libraries mark themselves safe with sideEffects: false in package.json. Importing { debounce } from a modular package instead of the whole library lets tree shaking work.
12
A third-party library you need has no TypeScript types. What do you do?
First I check if types exist on DefinitelyTyped by installing @types/library-name. If not, I write a minimal declaration file, for example declare module 'old-chart-lib' with only the functions I use, in a types folder included by tsconfig. This is better than a blanket any import. If the library is important to the community, contributing those types upstream helps everyone.
13
How do you handle unclear requirements from a product manager?
Situation: a product manager asked for 'offline support' in our web app without details. Task: I needed clarity before estimating. Action: I listed specific questions, which screens, read-only or editing, how long offline, and shared a quick prototype showing two options with effort estimates. Result: we agreed on read-only offline access for the order history, which shipped in one sprint instead of an open-ended project.
14
What is the difference between unit, integration and end-to-end tests?
A unit test checks a single function or module in isolation, such as a price formatter. An integration test checks several pieces working together, like a form component with validation and an API call mocked at the network level. An end-to-end test drives a real browser through a complete user flow, such as signup to dashboard, against a running app. They trade speed for realism in that order.
15
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.
16
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.
17
Write a card that changes layout based on its container width.
Use a container query so the card adapts wherever it is placed:
.card-wrapper {
container-type: inline-size;
}
.card {
display: grid;
gap: 1rem;
}
@container (min-width: 420px) {
.card {
grid-template-columns: 160px 1fr;
align-items: center;
}
}
In a narrow sidebar the image stacks above text; in a wide main column it sits beside the text, with no viewport media queries.
Component-driven development builds UIs from the bottom up as small, reusable, isolated components, such as buttons and inputs, composed into larger patterns like forms and pages. Each component has a clear API through props or attributes and is developed and documented in isolation, often in Storybook. Composition over configuration keeps components flexible. This approach enables design systems, parallel work and easier testing.
19
The build works on your machine but fails in CI. How do you troubleshoot?
I would compare environments first: Node version, package manager version, and whether CI uses npm ci with the committed lockfile while I used npm install. Other common causes are case-sensitive file paths on Linux, missing environment variables, and files ignored by Git. Pinning Node with an .nvmrc or engines field and reproducing the CI command locally in a clean clone or container usually reveals the difference.
20
Write a fetch wrapper that retries with exponential backoff.
Visual regression testing captures screenshots of pages or components and compares them pixel by pixel with approved baselines, flagging unexpected visual changes such as broken spacing or wrong colours. Tools include Playwright's toHaveScreenshot, Chromatic with Storybook and Percy. To avoid false failures, run in consistent containers, disable animations, and mask dynamic content like dates. It catches CSS bugs that functional tests miss.
22
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.
23
Search results sometimes show data for an older query. What is happening?
This is a race condition: requests for 'ja' and 'java' are in flight together, and if the older one responds last, it overwrites the newer results. I would cancel the previous request with AbortController, or tag each request with an id and ignore responses that are not the latest. Debouncing the input reduces the number of requests, and caching repeated queries makes responses faster and more consistent.
TypeScript enums generate runtime JavaScript objects, and numeric enums accept any number, which weakens safety. String literal unions like type Status = 'draft' | 'published' have zero runtime cost, work naturally with JSON and give the same autocomplete. When you need a runtime list of values, an as const object plus a derived type works well. Many teams avoid enums, especially since type-stripping tools do not support them.
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.
Logical properties describe layout relative to the writing direction instead of physical sides. margin-inline-start means left in English but right in Arabic or Urdu, and padding-block maps to top and bottom in horizontal text. Using them, with the dir attribute on html, lets one stylesheet support both left-to-right and right-to-left languages without overrides. Sizes like inline-size replace width in the same way.
27
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.
A tagged template is a function placed before a template literal, like html`<p>${name}</p>`. The function receives the static string parts as an array and the interpolated values separately, so it can escape, transform or validate values before joining them. Libraries use this pattern for safe HTML templating, CSS-in-JS and GraphQL queries. Normal template literals simply interpolate values and support multi-line strings.
29
Run linting and formatting automatically before each commit.
Use Husky with lint-staged:
npm i -D husky lint-staged
npx husky init
echo 'npx lint-staged' > .husky/pre-commit
// package.json
"lint-staged": {
"*.{ts,tsx,js}": ["eslint --fix", "prettier --write"],
"*.{css,md,json}": "prettier --write"
}
Only staged files are checked, so commits stay fast. CI should still run the full lint because hooks can be skipped locally.
30
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.
31
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.
32
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.
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.
34
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.
35
Create a type where all fields are optional except id.
The server probably sends a date like 2026-11-12 without time zone, and new Date('2026-11-12') treats date-only strings as UTC midnight, while other formats are parsed as local time, causing shifts around midnight. I would send full ISO timestamps with offsets, store times in UTC, and format for display with Intl.DateTimeFormat('en-IN', { timeZone: 'Asia/Kolkata' }). The Temporal API, now arriving in browsers, makes this safer.
any turns off type checking completely, so errors slip through silently. unknown also accepts any value, but you must narrow it with typeof, instanceof or a type guard before using it, which makes it the safe choice for API responses and catch errors. never represents values that never occur, such as a function that always throws, or the leftover case in an exhaustive switch over a union.
38
What are declaration files (.d.ts) and when do you write them?
Declaration files describe the types of JavaScript code without containing implementation. Libraries ship them so TypeScript users get type checking, and many come from DefinitelyTyped as @types packages. You write your own when using an untyped library, declaring global variables injected by scripts, or typing non-code imports like declare module '*.svg'. Generating them with declaration: true is standard when publishing a TypeScript library.
Snapshot tests save a component's rendered output and fail when it changes, which quickly detects unintended changes with little effort. However, large snapshots are hard to review, so developers often update them blindly, and they break on harmless markup changes. They work best for small, stable outputs like serialised config or error messages. For UI behaviour, explicit assertions on visible text and roles are more meaningful.
40
Write a minimal observable store in plain JavaScript.
A store holds state and notifies subscribers:
function createStore(initial) {
let state = initial;
const listeners = new Set();
return {
get: () => state,
set(update) {
state = { ...state, ...update };
listeners.forEach(fn => fn(state));
},
subscribe(fn) {
listeners.add(fn);
return () => listeners.delete(fn);
}
};
}
const cart = createStore({ items: 0 });
const stop = cart.subscribe(s => (badge.textContent = s.items));
cart.set({ items: 2 });
The returned unsubscribe function prevents memory leaks.
Frequently asked questions
What is asked in Frontend Development interviews for 3 years of experience?
Three-year candidates are expected to own features and influence code quality. Expect frontend system design questions like an autocomplete or dashboard, advanced TypeScript, caching strategies, security practices and CI setup. Interviewers also probe code review habits, mentoring juniors and how you work with designers and backend teams.
How many questions should I practise before a Frontend Development interview?
Work through all 40 questions here, then the company-wise sets for the employers you are targeting. Quality matters more than count: be able to explain why each answer is right.
Which topics matter most for 3 years of experience?
Focus first on Frontend system design, Advanced TypeScript, Security practices, Code review and mentoring.