Explain the Virtual DOM and Reconciliation algorithm

What is the Virtual DOM?

The Virtual DOM (VDOM) is a lightweight JavaScript representation of the actual DOM. It’s a programming concept where an ideal, or “virtual”, representation of a UI is kept in memory and synced with the “real” DOM.

Why Virtual DOM?

Direct DOM manipulation is slow because:

• DOM operations trigger reflow and repaint
• Batch updates are difficult to manage
• Performance bottlenecks in complex UIs

// Virtual DOM representation
const vdom = {
  type: 'div',
  props: {
    className: 'container',
    children: [
      { type: 'h1', props: { children: 'Hello' } },
      { type: 'p', props: { children: 'World' } }
    ]
  }
};

---

The Reconciliation Algorithm

Reconciliation is the process React uses to diff one tree with another to determine which parts need to be changed.

Key Principles

1. Different Component Types → Tear down old tree, build new one
2. Same Component Type → Update props, keep instance
3. Keys → Identify which items have changed, added, or removed

Diffing Algorithm

// Without keys - inefficient
<ul>
  <li>First</li>
  <li>Second</li>
</ul>

// React will mutate every child

// With keys - efficient
<ul>
  <li key="1">First</li>
  <li key="2">Second</li>
</ul>

// React knows which specific elements changed

---

Fiber Architecture

React 16 introduced Fiber, a complete rewrite of the reconciliation algorithm.

What Fiber Enables

• Incremental Rendering: Split work into chunks
• Pause, Abort, Reuse: Work can be paused and resumed
• Priority: Assign priority to different types of updates
• Concurrency: Prepare multiple versions of the UI

// Simplified Fiber node structure
const fiber = {
  type: 'div',
  key: null,
  props: { className: 'container' },
  
  // Relationships
  parent: parentFiber,
  child: firstChildFiber,
  sibling: nextSiblingFiber,
  
  // State
  alternate: previousFiber,
  effectTag: 'UPDATE',
  
  // Work tracking
  expirationTime: timestamp
};

Reconciliation Steps

1. Render Phase (can be interrupted)

   • Build work-in-progress tree
   • Calculate diffs
   • Mark effects

2. Commit Phase (synchronous)

   • Apply DOM changes
   • Run lifecycle methods
   • Update refs

---

Performance Optimization

Understanding reconciliation helps optimize React apps:

// ❌ Bad: Creates new object every render
<Component style={{ color: 'red' }} />

// ✅ Good: Stable reference
const style = { color: 'red' };
<Component style={style} />

// ❌ Bad: Inline function
<button onClick={() => handleClick(id)}>Click</button>

// ✅ Good: Memoized callback
const handleClick = useCallback(() => {
  doSomething(id);
}, [id]);
<button onClick={handleClick}>Click</button>

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