How does React's Virtual DOM work and what is the Reconciliation process in React 18 Fiber?

Quick Revision Summary

Virtual DOM is an in-memory JS representation. React Fiber reconciles differences using an O(n) diffing heuristic and batches updates to minimize browser reflows.

Interactive Practice Mode

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Model Answer & Deep-Dive Explanation

The Virtual DOM (VDOM) is an in-memory lightweight JavaScript object representation of the real browser DOM. Directly manipulating the browser DOM is expensive because every change triggers costly style recalculations, layout reflows, and repainting.

Reconciliation Process:

When component state or props change, React invokes the component function and generates a new Virtual DOM tree.
React performs a diffing algorithm comparing the new Virtual DOM with the previous snapshot.
Under React Fiber (the reconciliation engine since React 16), the reconciliation process is split into incremental, interruptible chunks. High-priority tasks (like user clicks or typing) can pause lower-priority background renders.
Only the exact minimal set of altered DOM nodes is patched to the real browser DOM in the final commit phase.

Interviewer Evaluation Points & Scoring Rubric

1 1. Explains why real DOM updates are expensive (reflow/repaint). 2. Describes the diffing and commit stages. 3. Explains Fiber's interruptible rendering architecture.

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How does React's Virtual DOM work and what is the Reconciliation process in React 18 Fiber?

Interviewer Evaluation Points & Scoring Rubric

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