Virtual DOM

The Virtual DOM is an in-memory representation of a UI tree that React uses to describe what the interface should look like. React can compare successive trees and determine which changes need to be committed to the real DOM.

In React, components produce elements that describe the desired UI. React represents this structure in memory rather than requiring application code to manipulate browser DOM nodes directly.

When state or props change, React renders the affected component tree again and produces a new element tree. React then reconciles the new tree with the previous one, using the differences to determine the necessary updates.

The Virtual DOM is not a second browser DOM. It is a JavaScript-level representation of UI structure. The browser DOM remains the actual structure used for rendering and interaction in the page.

Reconciliation can avoid unnecessary DOM operations, but the Virtual DOM should not be described as automatically making every React application faster. Performance depends on component structure, rendering work, reconciliation, and the resulting DOM updates.

Key points

• The Virtual DOM is an in-memory representation of UI structure.
• React renders new element trees when relevant state or props change.
• Reconciliation compares the previous and next trees to determine updates.
• The Virtual DOM is distinct from the browser's actual DOM.
• Using a Virtual DOM does not guarantee optimal performance by itself.

Interview questions

Is the Virtual DOM the same thing as the real DOM?

No. The browser DOM consists of actual nodes managed by the browser and displayed as part of the web page. The Virtual DOM is an in-memory representation of UI structure used by React. React uses its representations during rendering and reconciliation, then commits appropriate changes to the actual host environment. Therefore, manipulating a Virtual DOM representation is not equivalent to directly manipulating a browser DOM node.

What is the Virtual DOM?

The Virtual DOM is an in-memory representation of the UI structure used by React. Components produce descriptions of what the UI should contain. When relevant data changes, React can produce a new representation and compare it with the previous one. React then applies the necessary changes to the actual DOM.

Why does React use a Virtual DOM?

It gives React a declarative way to describe UI without requiring components to manually coordinate individual DOM mutations. React can compare the previous and next UI descriptions and determine which host changes are needed. This can reduce unnecessary DOM work in many applications. The main benefit is also architectural: application code describes the desired result rather than managing DOM mutations itself.

What happens when React state changes?

A state update schedules React to render the affected component tree. React obtains a new representation of the UI based on the new state. It then reconciles that result with the previous representation and determines the work required to bring the UI up to date. The necessary host changes are subsequently committed to the browser environment.

What role do keys play in reconciliation?

Keys give React a stable identity for elements among siblings, especially when rendering lists. During reconciliation, React can use those identities to determine which items correspond across renders. Stable keys help React preserve the appropriate component and DOM state when items are inserted, removed, or reordered. Using array indexes as keys can cause incorrect identity behavior when the list order changes.

Does the Virtual DOM guarantee better performance?

No. A Virtual DOM is an implementation strategy, not a blanket performance guarantee. React still has to perform rendering and reconciliation work, and poorly structured component trees can create unnecessary work before the browser DOM is updated. Performance depends on factors such as component boundaries, state placement, list identity, rendering cost, and the amount of host work committed.