The frontend developer interview has broadened far beyond "center this div." In 2026, frontend engineers are expected to know JavaScript deeply, reason about React rendering, write performant and accessible UI, and increasingly handle frontend system design. This guide covers the frontend interview questions that actually come up, organized by area, with what each is really testing.
JavaScript fundamentals
- Explain closures and give a real use case.
- What is the difference between == and ===, and between var, let, and const?
- How does the event loop work? Explain the microtask vs macrotask queue.
- What is hoisting? What is the temporal dead zone?
- Explain this binding, and the difference between arrow and regular functions.
- What are promises, and how do async/await differ from
.then()? - Implement debounce and throttle from scratch.
React (and component frameworks)
- What is the virtual DOM and how does reconciliation work?
- Explain useEffect — dependency arrays, cleanup, and common bugs.
- What causes unnecessary re-renders, and how do you prevent them (memo, useMemo, useCallback)?
- Controlled vs uncontrolled components — when to use each.
- How does state management scale — Context vs Redux vs signals?
- What are keys in lists and why do they matter?
CSS and layout
- Explain the box model and
box-sizing. - Flexbox vs Grid — when do you reach for each?
- How does specificity work, and how do you debug a stubborn override?
- What is the stacking context, and why does
z-indexsometimes "not work"? - How do you build a responsive layout without a framework?
Performance and accessibility
- What are the Core Web Vitals (LCP, INP, CLS) and how do you improve them?
- Explain code splitting, lazy loading, and tree shaking.
- How do you reduce bundle size and time-to-interactive?
- What makes a component accessible — semantic HTML, ARIA, keyboard navigation, focus management?
Frontend system design
Senior frontend interviews now include a design round: "design a typeahead search", "design an infinite-scroll feed", "design a component library." Drive it like any design question — clarify requirements, discuss data fetching and caching, state management, rendering strategy, error/loading states, and accessibility. Talk trade-offs out loud.
Practise explaining, not just memorizing
You can recognize all of these on a flashcard and still stumble when asked to explain closures out loud to an interviewer. The interview is verbal, so practice has to be verbal. Greenroom runs spoken frontend mock interviews, asks follow-ups on your real projects, and gives feedback on how clearly you explain technical concepts. Pair it with talking about your GitHub projects and coding-interview communication tips.
Frequently asked questions
What should I study for a frontend developer interview?
Focus on JavaScript fundamentals (closures, the event loop, promises, this binding), your framework (React rendering, hooks, re-render performance), CSS layout (flexbox, grid, specificity, stacking context), performance (Core Web Vitals, code splitting), accessibility, and increasingly frontend system design for senior roles.
What JavaScript questions come up most in frontend interviews?
The most common are explaining closures with a use case, how the event loop and microtask queue work, the differences between var, let, and const, how 'this' binding works, promises versus async/await, and implementing debounce and throttle from scratch.
Do frontend interviews include system design?
Yes, increasingly — especially for mid and senior roles. Frontend system design questions like 'design a typeahead search' or 'design an infinite-scroll feed' test how you handle data fetching, caching, state management, rendering strategy, loading and error states, and accessibility. Drive them with a clear framework and talk trade-offs out loud.
How do I prepare for the verbal part of a frontend interview?
Practise explaining concepts out loud, not just recognizing them on flashcards. Interviewers care whether you understand why something works, so rehearse explaining things like closures, re-render bugs, or a project decision verbally — ideally with a mock interview that asks follow-up questions.