How does React updates virtual DOM?

React relies on a process called reconciliation to update the virtual DOM and subsequently the real DOM. At the heart of reconciliation is a diffing algorithm that efficiently identifies the minimal changes required.

Here's a deeper dive into React's approach:

1. Heuristic Approach: Unlike some generic algorithms with high complexity, React utilizes a heuristic O(n) algorithm. This means the time to compare grows linearly with the number of elements (n). This efficiency is crucial for large applications.

2. Key Assumptions: The diffing algorithm makes two key assumptions:

   • Different Types, Different Trees: Elements of fundamentally different types (like <div> vs <img>) are assumed to have entirely different structures and hence lead to a full rebuild of the virtual DOM subtree.

   • Key Prop for Stability: Developers can provide a key prop for child elements. This prop helps React identify stable elements across renders, even if their content changes. This allows React to update only the content and avoid unnecessary re-renders of the entire element.

3. Diffing Steps: When comparing virtual DOM trees, React follows these general steps:

   • Root Element Comparison: It first compares the root elements. If the types differ (as mentioned earlier), a full rebuild happens.

   • Recursive Comparison: If the root elements are the same type (e.g., both are <div>s), React recursively compares their children.

   • Updates Based on Differences: Based on the identified differences, React determines how to update the real DOM:

     • Re-render and Replace: If elements have significant changes, they might be entirely re-rendered and replaced in the real DOM.

     • Attribute Updates: If only certain attributes change (e.g., a CSS class), React updates only those attributes in the real DOM.

     • Reordering: If the order of elements changes, React efficiently reorders them in the real DOM.

Remember: React prioritizes optimizations. If a change is complex or the key prop isn't effectively used, it might opt for a more conservative approach like a full rebuild for a subtree.

By leveraging this diffing algorithm, React minimizes unnecessary DOM manipulations, leading to a performant and responsive user experience in your web applications.