What is React's VDOM? And How Does the Virtual DOM Perform the Diff Algorithm?

React's Virtual DOM (VDOM) is one of the core concepts in React used to enhance application performance. It is a lightweight abstraction of the real DOM. Essentially, the Virtual DOM is a JavaScript object that represents a simplified version of the real DOM structure. React uses the Virtual DOM to simulate updates to the real DOM, thereby minimizing direct interactions with the real DOM, as real DOM operations typically incur significant overhead.

When the state of a component changes, React creates a new Virtual DOM tree and compares it with the previous Virtual DOM tree. This process is called the Diff algorithm. Through the Diff algorithm, React determines the minimal updates required for the actual DOM. The following are the key steps of the Diff algorithm:

1. Tree Comparison: React first compares the root nodes of the two trees. If the types of the root nodes differ (e.g., from <div> to <span>), React unmounts the old tree and mounts a new one. If the types are identical, it retains the root node and proceeds with recursive comparison.

2. Component Comparison: For React component nodes, React checks if the component type matches. If the type is the same, the component receives new props and re-renders. React then compares the returned Virtual DOM.

3. Child Element Comparison: When comparing elements of the same type, React continues to compare their child elements. React employs two strategies:

   • Same-Level Comparison: React only compares child elements at the same hierarchical level. Identical elements at different levels are not reused.
   • Key Attribute: When developers provide a key attribute, React uses this key to match elements between the old and new Virtual DOM trees. This preserves state and improves performance, especially in list rendering.

4. Updating the DOM: Once the Diff algorithm identifies the minimal changes, React executes these updates in batches to reduce real DOM interactions, thereby optimizing performance.

Example:

Consider a list where each <ListItem /> component has a unique key attribute, and a state update reverses the order of list items. Since each <ListItem /> has a unique key, React recognizes that the components have only changed order, not that they are entirely new. Thus, React only reorders the list items in the DOM, avoiding full destruction and recreation of the list, which significantly boosts performance.

Summary:

React's Virtual DOM and Diff algorithm collaborate to provide an efficient update mechanism. The Virtual DOM enables React to perform rendering and updates in memory, while the Diff algorithm ensures only necessary and minimal modifications are applied to the real DOM. This approach allows React to maintain high performance when handling large, dynamic applications.