What is Virtual DOM in React?

This article provides a clear and concise explanation of the Virtual DOM in React, detailing what it is, how it functions behind the scenes, and why it is crucial for building high-performance web applications. You will learn how React uses this programming concept to optimize rendering and update the user interface efficiently.

Understanding the Real DOM

To understand the Virtual DOM, it is helpful to first understand the Real DOM (Document Object Model). The DOM is an HTML document represented as a tree structure. When a web page loads, the browser creates this model to allow JavaScript to manipulate the page’s content, structure, and style.

However, updating the Real DOM is computationally expensive. When an element changes, the browser often has to recalculate the CSS, layout the page, and repaint the entire screen. In complex applications with thousands of elements, frequent updates to the Real DOM cause noticeable lag and a poor user experience.

What is the Virtual DOM?

The Virtual DOM is a lightweight, in-memory representation of the Real DOM. It is a programming concept where a “virtual” representation of the user interface is kept in memory and synced with the “real” DOM by a library such as ReactDOM. This synchronization process is called reconciliation.

The Virtual DOM has the exact same properties as the Real DOM, but it lacks the power to directly change what is on the screen. Because it does not write directly to the screen, updating the Virtual DOM is extremely fast.

How the Virtual DOM Works

React utilizes a specific, highly optimized workflow to update the user interface:

  1. State Change: When the state or props of a component change, React creates a new Virtual DOM tree representing the updated UI.
  2. Diffing: React then compares this new Virtual DOM tree with the previous Virtual DOM tree (the one created before the update). This comparison process uses a highly optimized “diffing” algorithm to identify exactly what has changed.
  3. Reconciliation: Once the differences (the “diffs”) are identified, React calculates the most efficient way to update the Real DOM.
  4. Batch Updates: Instead of updating the Real DOM for every single change immediately, React batches the changes. It applies only the necessary updates to the Real DOM in a single go, minimizing browser repaints and reflows.

Key Benefits of the Virtual DOM