Frontend Developer Interview Questions 2026
Questions hiring managers ask to test JavaScript fundamentals, UI architecture, and debugging instincts.
10 questions3 categoriesWith answer hints
Technical
4 questions1Explain the difference between `null` and `undefined` in JavaScript, and when you'd intentionally use each.
Hint: Cover type differences, JSON serialization behavior, and give a practical example like optional function params vs uninitialized state.
2What is the browser's critical rendering path, and how does it affect perceived load performance?
Hint: Walk through DOM → CSSOM → Render Tree → Layout → Paint. Mention render-blocking resources and how async/defer attributes help.
3How does React's reconciliation algorithm work, and what role do keys play in list rendering?
Hint: Explain the diffing heuristic (same type = update, different type = remount). Clarify that keys must be stable and unique among siblings, not globally.
4What are CSS custom properties (variables), and how do they differ from Sass variables?
Hint: Sass vars compile away; CSS custom properties are live in the DOM, can be scoped to elements, and are readable/writable via JS. Theming is the canonical use case.
Behavioral
3 questions5Tell me about a time you significantly improved the performance of a web page. What was the problem and how did you approach it?
Hint: Use STAR format. Quantify improvement (LCP, TTI, bundle size). Mention specific tools like Lighthouse, Webpack Bundle Analyzer, or Chrome DevTools.
6Describe a situation where you had to push back on a designer's spec because it would have caused accessibility or performance problems.
Hint: Show collaborative conflict resolution. Mention the specific issue (e.g., contrast ratio, animation on low-power devices) and the outcome reached.
7Tell me about a bug that took you longer than expected to find. What made it hard to reproduce?
Hint: Interviewers want process: how you isolated the failure, what tools you used, what you learned about defensive coding or logging as a result.
System Design
3 questions8Design a component library that supports both light and dark themes and can be used across multiple products.
Hint: Cover CSS custom properties/design tokens, theming strategy, component API design, Storybook documentation, and how to publish and version the library.
9How would you architect a real-time collaborative text editor in the browser (similar to Google Docs)?
Hint: Discuss Operational Transformation or CRDT algorithms, WebSocket connections, conflict resolution, presence indicators, and offline-first considerations.
10Design a client-side routing system from scratch. What edge cases must it handle?
Hint: Cover history API vs hash routing, nested routes, lazy loading, scroll restoration, 404 handling, and guarding authenticated routes.
Ready to prepare?
Study the Frontend Developer Roadmap
See the full step-by-step path — skills, timelines, and resources — so you can answer every question above with real experience behind it.
View Frontend Developer RoadmapQuestions reflect commonly asked interview topics for Frontend Developer roles across companies of various sizes. Hints summarize what strong answers typically cover — use them as a preparation guide, not a script.