25-Mar-2025

The Virtual DOM is a core concept in many modern JavaScript frameworks, most notably React. It’s a key factor in their performance and efficiency. Essentially, it’s an in-memory representation of the actual DOM (Document Object Model). Instead of directly manipulating the real DOM, which can be computationally expensive, changes are first made to this lightweight JavaScript representation. The framework then compares the Virtual DOM with a previous snapshot and determines the most efficient way to update the actual DOM based only on the differences (a process called “diffing” or “reconciliation”).

Here’s a simplified conceptual example using JavaScript:

// Simplified representation of a Virtual DOM node
function VNode(type, props, children) {
  this.type = type;
  this.props = props;
  this.children = children;
}

// Create a Virtual DOM representation
const oldVNode = new VNode('div', { id: 'container' }, [
  new VNode('p', { className: 'text' }, ['Hello']),
  new VNode('span', null, ['World'])
]);

const newVNode = new VNode('div', { id: 'container' }, [
  new VNode('p', { className: 'text' }, ['Hello']),
  new VNode('span', null, ['There']) // Change: World -> There
]);


// Simplified diffing algorithm (conceptual)
function diff(oldNode, newNode) {
  if (oldNode.type !== newNode.type) {
    // Replace the entire node
    console.log(`Replacing ${oldNode.type} with ${newNode.type}`);
    return;
  }

  if (oldNode.props !== newNode.props) {
    // Update properties
    console.log(`Updating props of ${oldNode.type}`);
    for (const prop in newNode.props) {
        if(newNode.props[prop] !== oldNode.props[prop]){
          console.log(`Changed prop ${prop} from ${oldNode.props[prop]} to ${newNode.props[prop]}`)
        }
    }
  }
  
  // Diff children recursively
  const oldChildren = oldNode.children || [];
  const newChildren = newNode.children || [];

  const maxLength = Math.max(oldChildren.length, newChildren.length);

  for (let i = 0; i < maxLength; i++) {
      if(!oldChildren[i]){
        console.log(`Adding a new child node: ${newChildren[i].type}`)
        continue
      }
      if(!newChildren[i]){
          console.log(`Removing a new child node: ${oldChildren[i].type}`)
          continue;
      }

    diff(oldChildren[i], newChildren[i]);
  }

}


diff(oldVNode, newVNode);
// Output demonstrates only the necessary change being detected
// Updating props of span
// Changed prop children from World to There

This simplified example demonstrates how a diffing algorithm would compare the two Virtual DOM trees and detect only the necessary changes. Real-world implementations are significantly more complex, handling various edge cases and optimizations, but the underlying principle remains the same: minimize direct DOM manipulations by batching updates. This leads to performance improvements, especially in complex and dynamic applications.