Explain how virtual DOM works?

Here's how the Virtual DOM works and how it aids in performance optimization in React:

1
Initial Render: When we create a React component, it generates a Virtual DOM representation of the component's structure and content. This Virtual DOM is essentially a JavaScript object tree, mimicking the actual structure of the HTML elements.
2
State and Props Changes: Whenever the state or props of a component change, React creates a new Virtual DOM representation of the component. This new Virtual DOM is then compared with the previous one.
3
Virtual DOM Reconciliation: React performs a process of reconciliation to compare the previous Virtual DOM with the new one. It identifies the differences between the two representations.
4
Minimal Updates: React calculates the minimal set of changes needed to bring the Virtual DOM in sync with the real DOM. Instead of directly updating the entire real DOM, React calculates and applies only the necessary changes.
5
Batching and Rendering: React batches multiple changes together and applies them in a single update to the real DOM. This reduces the number of times the real DOM is accessed and manipulated, resulting in better performance.

Difficulty: 5/10

Topics: reconciliation, diffing algorithm, batch updates

0-2 years experience

1
If you add a new item to a list in state, can you walk me through what React does with the virtual DOM to update the UI?
2
Suppose you directly mutate a component's state without using setState; what would happen to the virtual DOM diffing process?
3
How would you force a re‑render of a component without changing its props or state, and what effect does that have on the virtual DOM?

2-5 years experience

1
We have a component that renders a large table; after a recent change, the UI is flickering. How would you investigate whether the virtual DOM diffing is causing the issue?
2
When implementing optimistic UI updates, how do you ensure the virtual DOM correctly reconciles the temporary state with the server response?
3
If a child component re‑renders even though its props haven't changed, what virtual DOM mechanisms could be responsible and how would you fix it?

5-8 years experience

1
Design a custom hook that batches multiple state updates to minimize virtual DOM re‑renders. What considerations around the diffing algorithm and scheduling would you keep in mind?
2
In a high‑traffic dashboard rendering thousands of items, how would you leverage the virtual DOM together with windowing or memoization to keep performance acceptable?
3
Explain how you would approach migrating a legacy React app that uses manual DOM manipulation to rely fully on the virtual DOM, addressing potential pitfalls.

8+ years experience

1
Our organization is evaluating building a custom React renderer for a native platform. What architectural implications does the virtual DOM have for such a renderer, and how would you design the reconciliation layer?
2
When planning a long‑term strategy for a large codebase, how would you decide between staying with React's virtual DOM approach versus adopting a different UI paradigm like Web Components or Svelte, considering maintainability and performance?
3
If we need to support server‑side rendering with streaming and client hydration across multiple teams, what challenges does the virtual DOM introduce and how would you coordinate a solution?

❓How does the key prop influence the diffing process?
❓What differences would you expect if you bypassed the virtual DOM and manipulated the DOM directly?
❓In what ways does React's concurrent mode affect virtual DOM updates?