Virtual DOM Interview Questions
February 26, 2026 • By Surya Singh • React • Virtual DOM • Frontend • Interview
Virtual DOM interview questions — reconciliation, diffing algorithm, React Fiber.
Key Takeaways
- 1Virtual DOM is an in-memory representation of the UI; React compares it to the previous tree (diffing) and applies minimal DOM updates.
- 2Reconciliation is the algorithm that compares two trees; it uses heuristics (same type, key) for O(n) complexity.
- 3Keys must be stable and unique; avoid using array index as key when list order changes.
- 4The Virtual DOM enables declarative UI and batching; the benefit is developer experience and predictable updates, not raw speed.
The questions below are commonly asked in technical interviews. Each answer is written to help you understand the concept clearly and explain it confidently. Focus on understanding the "why" behind each answer—that is what interviewers care about.
In this guide
Interview Questions & Answers
What is the Virtual DOM and why does React use it?
The Virtual DOM is a lightweight JavaScript representation of the real DOM. When your state changes, React builds a new virtual tree, compares it to the previous one (a process called reconciliation or diffing), and computes the minimal set of DOM updates needed. Then it applies those updates in a batch. The Virtual DOM enables a declarative style: you describe what the UI should look like, and React figures out how to update the DOM efficiently. It also allows React to batch updates and prioritize work (e.g., with concurrent features).
How does React's reconciliation algorithm work?
React compares the old and new virtual trees. If two elements have different types (e.g., div vs span), React replaces the whole subtree. If they have the same type, React keeps the DOM node and updates only the changed attributes. For children, React iterates and matches by key. If keys match, it updates in place; if a key is new or missing, it mounts or unmounts. This heuristic gives O(n) complexity instead of O(n³) for a full tree diff. Keys should be stable (same item = same key) and unique among siblings.
Why are keys important and when should I not use the index?
Keys help React identify which items changed, were added, or were removed. With good keys, React can reuse DOM nodes correctly. Using the array index as key is fine when the list is static (no reorder, no add/remove in the middle). But if you reorder or remove items, indices shift—React might reuse the wrong DOM node and show incorrect state or cause bugs. Use a unique ID from your data (e.g., item.id). If you have no ID, generate one when the list is created.
Is the Virtual DOM faster than direct DOM manipulation?
Not necessarily. The Virtual DOM adds overhead: building the tree, diffing, and then updating. For a single targeted update, direct DOM manipulation can be faster. The benefit of the Virtual DOM is that it makes declarative UI practical and enables batching—without it, you would manually track what changed and update the DOM, which is error-prone. React's reconciliation and batching reduce the number of DOM operations and layout thrashing. The real win is developer productivity and predictable behavior, plus optimizations like concurrent rendering.
What is the Fiber architecture in React?
Fiber is React's reconciliation engine (since React 16). Each component is represented as a fiber—a unit of work. Fibers form a tree, and React can traverse and update them incrementally. This enables features like concurrent rendering: React can pause work, yield to the browser for high-priority updates (e.g., user input), and resume later. Before Fiber, rendering was synchronous and blocking. With Fiber, React can split work into chunks and prioritize, which improves perceived performance for large component trees.
Loading...
Related Interview Guides
- All Frontend Mastery (React/JS) Topics
- System Design Interview Questions
- React Interview Questions
- SQL Interview Questions
- Interview Preparation — Start Here
Surya Singh
Azure Solutions Architect & AI Engineer
Microsoft-certified Azure Solutions Architect with 8+ years in enterprise software, cloud architecture, and AI/ML deployment. I build production AI systems and write about what actually works—based on shipping code, not theory.
- Microsoft Certified: Azure Solutions Architect Expert
- Built 20+ production AI/ML pipelines on Azure
- 8+ years in .NET, C#, and cloud-native architecture