ReactJS Reconciliation

Last Updated : 13 Aug, 2025

Reconciliation is the process React uses to figure out how to efficiently update the DOM (Document Object Model) when changes occur in the UI. React's goal is to update the page as efficiently as possible, without unnecessary re-rendering or slow performance.

Reconciliation helps in the following ways:

Minimizes unnecessary updates: React only changes the parts of the UI that actually need to be updated, rather than re-rendering the entire page.
Improves performance: By optimizing the update process, React reduces the number of changes to the actual DOM, which improves the performance of your application.
Ensures consistency: React ensures that the UI always matches the current state of your application, even as it changes over time.

How ReactJS Reconciliation Works

The reconciliation process involves the following steps:

1. Render Phase

React calls the render() method of a component to generate a new virtual DOM representation.
This new Virtual DOM is compared with the previous Virtual DOM snapshot.

2. Diffing Algorithm

React compares the old and new virtual DOM trees to determine the differences.
Instead of re-rendering the entire UI, React updates only the changed nodes.

3. Commit Phase

Once the differences are determined, React applies the updates to the real DOM in the most efficient way.
React batches updates and minimizes reflows and repaints for better performance.

Virtual DOM in React

The Virtual DOM (VDOM) is a lightweight, in-memory representation of the actual DOM elements.

It allows React to perform DOM updates more efficiently by updating only the necessary elements, avoiding full re-renders of the entire UI.
The Virtual DOM is a concept borrowed from the idea of "virtualization" in computing, where operations are performed on a virtual version of an object (like the DOM) before applying those changes to the actual object. This results in a more optimized and less resource-intensive approach.

How Virtual DOM Works

Initial Rendering: React creates an initial Virtual DOM tree when the components are first rendered. React then compares the Virtual DOM with the actual DOM and updates the actual DOM only where necessary.
State/Props Changes: When the state or props of a component change, React creates a new Virtual DOM tree. React then compares the new Virtual DOM with the previous one, determining what parts of the actual DOM need to be updated.
Efficient DOM Updates: React uses the diffing algorithm to identify the differences between the new and previous Virtual DOM trees, updating only the parts of the DOM that have changed.

Diffing Algorithm and Its Role in Reconciliation

The Diffing Algorithm plays a crucial role in the Reconciliation process. It is responsible for

Identifying Differences: The diffing algorithm identifies the differences between the old and new Virtual DOM trees by comparing each element.
Minimizing DOM Changes: The algorithm ensures that only the minimal number of changes are applied to the actual DOM.
Optimization: The diffing algorithm optimizes updates by reusing elements where possible and only making necessary changes.

Strategies for Optimizing ReactJS Reconciliation

1. Use shouldComponentUpdate

In class components, React provides the shouldComponentUpdate() lifecycle method, which allows us to prevent re-renders if the component’s state or props haven’t changed.

index.js

import React from 'react';

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0,
    };
  }

  // Only re-render if count is an even number
  shouldComponentUpdate(nextProps, nextState) {
    console.log('shouldComponentUpdate called');
    if (nextState.count % 2 === 0) {
      return true;  // Allow re-render
    }
    return false;   // Skip re-render
  }

  increment = () => {
    this.setState((prevState) => ({
      count: prevState.count + 1
    }));
  };

  render() {
    console.log('Render called');
    return (
      <div>
        <h2>Count: {this.state.count}</h2>
        <button onClick={this.increment}>Increment</button>
      </div>
    );
  }
}

export default Counter;

Output:

2. Use React.memo

React.memo is a higher-order component for functional components that prevents re-renders if the props haven't changed. It’s similar to shouldComponentUpdate but easier to use with functional components.

index.js

import React, { useState } from 'react';

// Child component wrapped in React.memo
const DisplayCounter = React.memo(({ count }) => {
  console.log('DisplayCounter rendered');
  return <h2>Count: {count}</h2>;
});

function App() {
  const [count, setCount] = useState(0);
  const [text, setText] = useState('');

  return (
    <div>
      <DisplayCounter count={count} />
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <input
        type="text"
        placeholder="Type something"
        value={text}
        onChange={(e) => setText(e.target.value)}
      />
    </div>
  );
}

export default App;

Output:

3. Use key Prop Efficiently in Lists

When rendering lists of elements, always use a unique key for each item. This helps React efficiently track and update individual elements without reordering or re-rendering the entire list.

JavaScript

import React from 'react';
const users = [
  { id: 101, name: 'Alice' },
  { id: 102, name: 'Bob' },
  { id: 103, name: 'Charlie' },
];

function UserList() {
  return (
    <ul>
      {users.map((user) => (
        <li key={user.id}>
          {user.name}
        </li>
      ))}
    </ul>
  );
}

export default UserList;

Output

4. Avoid Inline Functions and Objects in JSX

Inline functions and objects in JSX create a new instance on every render, which can cause unnecessary re-renders. Instead, define functions and objects outside of the render cycle or use useCallback to memoize them.

5. Use React.PureComponent

React.PureComponent is a base class for class components that implements shallow comparison of props and state to prevent unnecessary re-renders.

JavaScript

import React from 'react';
// PureComponent to avoid re-rendering if props haven't changed
class Display extends React.PureComponent {
  render() {
    console.log('Display re-rendered');
    return <h2>Count: {this.props.count}</h2>;
  }
}c

class App extends React.Component {
  state = {
    count: 0,
    dummy: '',
  };

  increment = () => {
    this.setState({ count: this.state.count + 1 });
  };

  updateDummy = () => {
    this.setState({ dummy: 'Changed' }); // Won't affect Display if count doesn't change
  };

  render() {
    return (
      <div>
        <Display count={this.state.count} />
        <button onClick={this.increment}>Increment</button>
        <button onClick={this.updateDummy}>Change Dummy</button>
      </div>
    );
  }
}

export default App;

Output

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Article Tags :

ReactJS Reconciliation

How ReactJS Reconciliation Works

1. Render Phase

2. Diffing Algorithm

3. Commit Phase

Virtual DOM in React

How Virtual DOM Works

Diffing Algorithm and Its Role in Reconciliation

Strategies for Optimizing ReactJS Reconciliation

1. Use shouldComponentUpdate

2. Use React.memo

3. Use key Prop Efficiently in Lists

4. Avoid Inline Functions and Objects in JSX

5. Use React.PureComponent

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