What is Reconciliation in React?
💡 Concept Name
Reconciliation – The process React uses to update the DOM by comparing the new virtual DOM with the previous one and applying the minimal number of changes.
📘 Quick Intro
When the state or props of a React component change, React generates a new virtual DOM tree. It then compares this tree with the previous one and efficiently updates only the parts of the real DOM that changed. This process is called reconciliation.
🧠 Analogy / Short Story
Imagine editing a Word document. Instead of rewriting the whole file, you just make changes where needed — fixing a word here, deleting a sentence there. That’s how React's reconciliation works: update only the changed parts, not the whole page.
It’s like a mail sorter comparing two lists: last week’s and this week’s. Only the differences are picked out and updated — that's how React spots DOM changes between renders.
🔧 Technical Explanation
- 📄 Reconciliation compares the new virtual DOM with the old one.
- ⚙️ React uses a diffing algorithm to identify minimal changes needed.
- 📦 Updates are batched and applied efficiently to the actual DOM.
- 🔑 Keys help React identify elements between renders and optimize updates.
- 🚀 This process ensures performance and keeps rendering fast and smooth.
💻 Real Code Example
import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<h1>{count}</h1>
<button onClick={() => setCount(count + 1)}>Increment</button>
</div>
);
}
❓ Interview Q&A
Q1: What is reconciliation in React?
A: It’s the process React uses to compare virtual DOM trees and apply minimal changes to the real DOM.
Q2: Why is reconciliation important?
A: It improves performance by avoiding full DOM re-renders.
Q3: What algorithm does React use in reconciliation?
A: A simplified diffing algorithm optimized for tree structures.
Q4: How do keys help in reconciliation?
A: They uniquely identify elements to help React track changes between renders.
Q5: When does reconciliation happen?
A: Whenever state or props change in a component.
Q6: Can reconciliation be customized?
A: To an extent, using keys, memoization, or by overriding lifecycle methods like shouldComponentUpdate.
Q7: What happens if keys are not used correctly?
A: React may re-render or misidentify elements, leading to UI bugs.
Q8: How does reconciliation differ from rendering?
A: Rendering creates the virtual DOM; reconciliation determines what changes to apply to the real DOM.
Q9: How does reconciliation affect performance?
A: It improves efficiency by limiting DOM operations.
Q10: What is the virtual DOM’s role in reconciliation?
A: It's the blueprint React uses to detect changes and update the real DOM accordingly.
📝 MCQs
Q1. What is reconciliation in React?
- Routing logic
- Data fetching
- Comparing virtual DOM trees and updating the real DOM
- Error handling
Q2. What does React compare during reconciliation?
- Props and state
- Event handlers
- New and old virtual DOM trees
- Redux state
Q3. What helps React identify elements between renders?
- Props
- Refs
- Keys
- State
Q4. Why is reconciliation fast?
- It avoids updates
- It compiles JSX
- It only updates changed elements
- It renders the full page
Q5. What causes reconciliation to trigger?
- Static methods
- Class decorators
- State or props change
- File imports
Q6. What happens if keys are duplicated?
- Nothing
- Optimized update
- Faster render
- React may misidentify or re-render elements incorrectly
Q7. What is virtual DOM?
- Browser memory
- JavaScript object
- A lightweight copy of the actual DOM
- Server cache
Q8. Which of the following helps React optimize reconciliation?
- Random IDs
- Re-renders
- Stable and unique keys
- Unused variables
Q9. How does React apply changes after reconciliation?
- Rebuilds all DOM nodes
- Calls external APIs
- It updates only necessary DOM nodes
- Disables re-renders
Q10. What is the role of diffing in reconciliation?
- Rebuild CSS
- Log errors
- It finds differences between old and new virtual DOM
- Create new pages
💡 Bonus Insight
React assumes elements of the same type have similar structures, allowing it to skip deep comparisons. Using stable keys and minimizing unnecessary changes helps React perform reconciliation more efficiently.
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