What is the virtual DOM in React?

The **virtual DOM** in React is an in-memory lightweight representation of the actual DOM (Document Object Model) used by browsers to render web pages. Instead of directly manipulating the real DOM, React builds a virtual DOM tree—a JavaScript object that mirrors the structure of the real DOM elements. ### How the Virtual DOM Makes React Apps Fast Manipulating the real DOM is relatively slow because it involves layout calculations, style recalculations, and repainting on the browser. React optimizes performance by minimizing these costly operations. When the state of a React component changes, React updates the virtual DOM first rather than the real DOM. React then performs a **diffing algorithm** to compare the new virtual DOM tree with the previous one. This process identifies exactly what has changed—whether elements were added, removed, or updated. After this efficient comparison, React batches and applies only the necessary changes to the real DOM. This selective update avoids unnecessary re-rendering and keeps the app responsive. ### What Happens During Reconciliation Reconciliation is the process by which React updates the UI to match the latest state. It involves: 1. **Rendering**: React calls the component’s render method (or function component) to produce a new virtual DOM tree. 2. **Diffing**: React compares the new virtual DOM tree with the previous one to spot changes at the element level. 3. **Applying Updates**: React generates a minimal set of DOM mutations needed to bring the real DOM in sync with the new virtual DOM. 4. **Batching**: These changes are grouped and executed together to avoid multiple reflows and repaints. Through reconciliation, React efficiently updates only the parts of the UI that have changed, rather than reloading the entire page or large parts of the DOM. This selective rendering is a key reason why React apps feel fast and performant. --- In summary, the virtual DOM acts as an intermediate, fast-updating layer that allows React to precisely and efficiently update the browser’s UI with minimal overhead. This mechanism, combined with reconciliation, ensures smooth and performant user experiences.