React FAQ: Top 75 Questions

The **Virtual DOM (VDOM)** is a programming concept where a virtual representation of the UI is kept in memory and synced with the "real" DOM by a library like ReactDOM. It's a lightweight copy of the actual DOM, stored as a JavaScript object.

React uses the Virtual DOM to optimize rendering performance and provide a more efficient way to update the browser's UI. Here's how it works:

• Initial Render: When a React component first renders, React creates a Virtual DOM tree (a JavaScript object representing the UI structure). This VDOM is then used to build the actual browser DOM.
• State/Prop Changes: When a component's state or props change, instead of immediately updating the real DOM, React creates a new Virtual DOM tree for the updated UI.
• Diffing Algorithm: React then runs a "diffing" algorithm to compare the new Virtual DOM tree with the previous one. This algorithm efficiently identifies only the specific differences between the two trees.
• Reconciliation: Based on these differences, React performs a process called "reconciliation." It calculates the most efficient way to update the real browser DOM.
• Batch Updates: React updates the real DOM with these calculated changes in a batched manner, minimizing direct manipulations to the browser's DOM. Direct DOM manipulation is generally slow and resource-intensive, so reducing its frequency significantly improves performance.

The key benefit is that React avoids re-rendering the entire actual DOM whenever there's a change, instead performing targeted, optimized updates. This leads to faster and smoother user interfaces.

import React, { useState, useEffect } from 'react';
import ReactDOM from 'react-dom/client';

function Counter() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('Real DOM might update now if count changed.');
    // In a real scenario, this log conceptually represents the point
    // where React applies changes to the actual DOM after VDOM diffing.
    // We don't directly manipulate the DOM here.
  }, [count]); // This effect runs whenever 'count' changes

  const increment = () => {
    setCount(prevCount => prevCount + 1);
  };

  const decrement = () => {
    setCount(prevCount => prevCount - 1);
  };

  return (
    <div>
      <h2>Count: {count}</h2>
      <button onClick={increment}>Increment</button>
      <button onClick={decrement}>Decrement</button>
    </div>
  );
}

const root = ReactDOM.createRoot(document.getElementById('root-vdom'));
root.render(<Counter />);
                    

Explanation of the Code:

• The `Counter` component maintains its `count` state.
• When `setCount` is called (via Increment/Decrement buttons), React doesn't immediately re-render the entire browser DOM.
• Instead, it updates its internal Virtual DOM representation of the `Counter` component.
• React then compares this new VDOM with the previous one, identifies that only the `count` display within the `h2` needs updating, and efficiently updates just that part of the real DOM. The `useEffect` with `console.log` is a conceptual placeholder to show that React's internal process for updating the real DOM happens *after* the state change.

This illustrates how React uses the VDOM to abstract away direct and often inefficient DOM manipulations, leading to optimized updates.