Deep dive into React's virtual DOM and reconciliation

Anton Ioffe - September 26th 2023 - 18 minutes read

In this illuminating exploration, we delve deep into the hidden layers of React, specifically focusing on the intricacies of its Virtual DOM and reconciliation process. React, emblematic of cutting-edge web development, has brought about a significant paradigm shift in the nuisances of rendering behavior. The Virtual DOM, an essential facet of React, has transformed the way the development community conceptualizes and interacts with the Document Object Model, paving the way for enhanced performance and efficiency.

As we navigate this enlightening journey, we will contrast the unique roles of Virtual DOM and Real DOM, explore the core mechanics of React's Virtual DOM, and how it, together with the reconciliation process, optimizes UI updates. We will dive into the unexplored territories of DOM updates, the importance of keys in performance optimization, and introduce you to the enthralling universe of Concurrent Mode and React Fiber.

This expansive article aims to provide senior-level developers an in-depth understanding of these complex topics supplemented with real-world use cases. So, buckle up for a deep dive into the mechanics of React's Virtual DOM and reconciliation process. It's time to unravel the magic that powers those lightning-fast, dynamic web interfaces!

Comparing the Realms of VirtualDOM and Real DOM

In the world of web development, the Document Object Model (DOM) serves as the programming interface for HTML and XML documents. It represents the structure of web documents in a tree-like format where each node represents a part of the document. However, direct manipulations of the real DOM can be quite expensive in terms of performance.

Enter the Virtual DOM - an abstraction of the real DOM, introduced and utilised by React to optimise and minimise real DOM manipulations. React's Virtual DOM serves as a lightweight copy of the real DOM, which represents the current state of the user interface (UI).

Distinctive Roles and Functions

So, what sets these two DOMs apart?

Performance: One of the key differences is the performance cost associated with each. Each time you update the real DOM, it triggers a reflow, repaint, or compositing, which can slow down the application. On the other hand, React's Virtual DOM conducts fewer operations by batching multiple updates into a single re-render, thereby optimising performance. To visualize the stark difference, consider an application where thousands of elements are being updated. With the real DOM, each change would trigger its own reflow or repaint, leading to significant slowdowns. On the contrary, Virtual DOM would batch these updates into one, greatly enhancing performance.

// Performance implication of multiple updates in Real DOM vs Virtual DOM
console.time('Real DOM');
for(let i = 0; i < 1000; i++) {
    let element = document.getElementById(`element${i}`);
    element.innerHTML = `Updated content ${i}`; 
console.timeEnd('Real DOM');

console.time('Virtual DOM');
let virtualElements = [];
for(let i = 0; i < 1000; i++) {
    virtualElements.push(React.createElement('div', {id: `element${i}`}, `Updated content ${i}`));
ReactDOM.render(virtualElements, document.getElementById('parentID')); 
console.timeEnd('Virtual DOM');

Updating Mechanism: In the real DOM, updates are immediate and direct, reflecting on the webpage instantly. However, in the Virtual DOM, updates are first conducted on the in-memory data-structure, which then undergoes a 'diffing' process to detect changes compared to the previous state of the Virtual DOM. After detecting changed nodes, only these differing nodes get updated in the real DOM, leading to optimised updates (referred to as reconciliation).

Flexibility, Control, Scalability, and Debugging: With the real DOM, control can be a bit cumbersome as updates happen immediately and can't be paused, stopped, or rearranged. Virtual DOM, however, provides tremendous flexibility in controlling and optimising updates. You can even delay updates to the real DOM during periods of high frequency updates to reduce performance costs. Moreover, Virtual DOM supports larger applications efficiently, enhancing their scalability through its batch update mechanism. And with the abstraction layer added by Virtual DOM, debugging becomes easier—React dev tools highlight Virtual DOM changes, providing discernible insight.

Consider these code examples:

// Real DOM update
let element = document.getElementById('id');
element.innerHTML = 'New Content'; // Directly updates the real DOM

// Virtual DOM update
let virtualElement = React.createElement('div', {id: 'id'}, 'New Content');
ReactDOM.render(virtualElement, document.getElementById('parentID')); 
// The Virtual DOM gets updated first. The real DOM update occurs only when the changes are committed.

In the above virtualElement example, changes are first made to the Virtual DOM. The reconciliation process then compares the updated Virtual DOM with the previous snapshot, after which the real DOM gets updated with the calculated differences.

Common Coding Mistakes

With this understanding, let's highlight potential pitfalls developers might encounter when working with React's Virtual DOM.

A common mistake is direct manipulation of the Virtual DOM instead of utilising React's setState() method. This practice is not recommended because it bypasses React's lifecycle and state management mechanisms that allow batch updates and optimised renders. When you manipulate the Virtual DOM directly, React is not aware of these changes and won't trigger an update to the real DOM.

// Incorrect - Direct Virtual DOM manipulation
this.state.value = 'New value'; 

// Correct - Utilising React's setState()
this.setState({value: 'New value'});

In the incorrect example, the component won't re-render because React won't know the state has changed. Always use setState() because it informs React about the state change, triggering an update to the Virtual DOM and, subsequently, the necessary updates to the real DOM.

Another common misconception among developers is expecting immediate changes in the component state after setState() is called. This misconception occurs because setState() is asynchronous, and updates to the state occur in batches for performance optimisation.

// Incorrect - Expecting immediate state change
this.setState({value: 'New value'});
console.log(this.state.value); // This may log the old value

// Correct - Accessing state after setState()
this.setState({value: 'New value'}, () => {
    console.log(this.state.value); // This will always log the new value

In the correct example, to access the updated state immediately after setState(), you should use a callback function.

As we delve deeper into React's Virtual DOM and debugging techniques, reflect upon this foundational knowledge. Consider how the Virtual DOM and real DOM operate distinctly. What could potential consequences be if the Virtual DOM was absent and React had to resort to direct updates on the real DOM?

React's Virtual DOM and The Reconciliation Process: Driving Efficiency

React's Virtual DOM is a core construct that aids in achieving optimum efficiency by enabling the batching and scheduling of updates to the User Interface (UI). Conversion of JSX components into a lightweight representation of the User Interface (UI) forms this Virtual DOM. Not as heavy as the real DOM nodes, these JavaScript objects provide a simple abstraction of the actual DOM and the changes it undergoes.

React's abstraction layer, the Virtual DOM, plays a significant role in enhancing performance. It does this by minimizing direct interaction with the actual DOM, which in turn reduces the cost of frequent updates.

However, the Virtual DOM is only part of the puzzle. The other key player in this performance boost is React's reconciliation process. Reconciliation is the mechanism by which React updates the UI based on changes made in a component's elements. It involves a diffing algorithm that compares the new Virtual DOM with the old one which identifies the minimal set of changes required in the actual DOM.

Diffing in the Virtual DOM

Diffing is a critical aspect of reconciliation. When a component's state or properties change, React creates a new Virtual DOM and then compares it with the old Virtual DOM. This comparison process is known as diffing. The result of this process is a collection of differences, or 'diffs', which React then uses to update the actual DOM.

However, diffing isn't a simple process, especially considering the potential number of nodes in the DOM tree. A naïve, brute force comparison could have an algorithmic complexity of O(N^3), which is not feasible for large DOMs. React optimizes this diffing process by making two assumptions:

  1. Two elements of different types will produce different trees.
  2. The developer can hint at which child elements may be stable across different renders with a key prop.

Armed with these assumptions, React applies heuristics to the diffing process to bring down the complexity to O(N).

‘Diffs’ in the List of Components

React uses keys to track individual elements across re-render cycles. Whenever keys are used, React keeps those elements associated with those keys. So, it not only minimally updates the existing component instances but also reuses them by rearranging. This approach significantly reduces the update time in case of reordering or adding or removing components.


After diffing, reconciliation is the next step in updating the UI. React iteratively walks through both trees - old and new - at the same time and generates mutations to the render tree that will be effective in updating the UI. This flow minimizes resource consumption in updating the real DOM, therefore, boosting performance.

State Manipulation and Application’s Rendering behavior

State manipulation, within React, plays a pivotal role in defining application rendering behaviour. Each state change fires a re-render. However, not all re-renders result in a change in the rendered output.

The primary question here arises that how React distinguishes between necessary and unnecessary renders? Here’s where the reconciliation power shows up. During the reconciliation process, React components that return the same output results from renders, virtual DOM objects don’t get a change status. This feature saves unnecessary renders, unmounts and mounts, thus making the updating cycle efficient and responsive.

In conclusion, Reconciliation and the Virtual DOM work side by-side to make React highly performant in the face of UI updates. These two concepts combined provide developers with a powerful abstraction, which reduces the direct interaction between JavaScript and the actual DOM. With a precise diffing process and efficient reconciliation, React proves to be a highly efficient tool for modern web development. The deep synergy between the Virtual DOM and reconciliation is crucial to understanding the inner workings of React and mastering its best practices.

Unpacking Efficiencies: DOM Updates and Mutations Across React and Traditional Approaches

In web development, manipulating the Document Object Model (DOM) is a necessary routine. A common approach to improving the experience of users is to minimize the amount of DOM manipulation, given that this can be an expensive process in terms of performance. This is where React, with its virtual DOM and reconciliation algorithm stands out, offering superior operating efficiency over conventional methods.

Conventional Methods: Direct DOM Operations

Traditionally, updating the DOM involves direct manipulations. For instance, when updating an HTML element, one could use a function like document.getElementById('') to locate and then alter the targeted element. This approach may seem straightforward, but its simplicity turns into a downside when it comes to larger processes with more DOM updates.

Every update would trigger a reflow or repaint, draining resources and potentially causing a noticeable lag in the user interface. Further, addressing the right component of an intricate web interface for modification can be a challenge.

// conventional method: direct DOM manipulation
document.getElementById('myId').textContent = 'New Value'; 

In this approach, even small changes incur a reflow or repaint, leading to a performance cost. For larger interfaces with more components, the cost is even heavier.

React's Approach: Virtual DOM and Batching

React uses a different approach to handle DOM updates. Instead of directly manipulating the DOM, it introduces the concept of a virtual DOM. The virtual DOM is a lightweight copy of the real DOM. Changes made to the web interface are first made in this virtual DOM, which is faster because it doesn't involve any painting or layout operations.

React also optimizes DOM updates by using a technique known as 'batching'. Rather than applying changes as soon as they are made, React batches multiple updates into a single update.

// React's approach: virtual DOM and batching
this.setState({ myValue: 'New Value' });

React then uses its reconciliation algorithm to effectively locate and batch the necessary changes. The changes are applied in one go to virtual DOM, minimizing the number of expensive DOM operations.

The Power of Diffing Algorithm

Apart from batching, React uses a diffing algorithm, comparing new and old versions of the virtual DOM to identify differences. Only the parts that have changed are updated in the real DOM.

This targeted approach lends to a more efficient process, minimizing redundant operations. Large applications with multiple components gain the most from this mechanism, striking an optimal balance between update needs and performance output.

// React's diffing in action
class MyComponent extends React.Component {
render() {
    return (
            <h1> Hello, world! </h1>
            <p> Time: {this.props.time} </p>

In the above piece of code, when this.props.time changes, only the text node inside <p> will be affected, ensuring minimal work is performed for maximum output efficiency.

React's dynamic approach to managing updates sheds light on the notion of complexity in coding. Managing a large number of updates might be complex but React simplifies this with its batch update and diffing algorithm. These algorithms collectively ensure that the re-rendering and updating of components don't drain the application's performance.

In conclusion, React handles DOM manipulations in a way that reaps maximum performance benefits. It employs a virtual DOM and utilizes an efficient reconciliation algorithm to batch updates, apply them in the virtual world first, and finally only update the necessary components in the real DOM. Understanding these technical details is fundamental in making full use of React's potential and producing high-performing web applications.

Here are some thought-provoking questions:

  1. How can we further optimize the performance of our React app beyond what virtual DOM and reconciliation provide?

  2. What are some potential scenarios where traditional DOM manipulation might still be more suitable?

  3. What trade-offs are we making when choosing React's approach over direct DOM manipulation and how do these impact the overall application performance?

By understanding in-depth how React operates, developers can make more informed decisions in their application design and choice of technologies.

Augmenting DOM Updates with Unique Keys: A Performance Primer

As we delve deeper into React's internal workings, the role of unique keys in augmenting React's updating mechanism within the Virtual DOM becomes clearer. Keys hold an underrated but pivotal role in the Virtual DOM reconciliation process that often gets overlooked despite their significant impact on performance.

Keys and the Reconciliation Process

React makes use of keys to identify each component uniquely in the DOM tree structure. From a performance standpoint, keys serve as pointers that React’s diffing algorithm uses during the reconciliation process to identify which items have changed, added, or removed.

For instance, consider a list or an array where each element corresponds to a DOM node. If the list gets modified, without keys, React's reconciliation process would have to resort to a more tedious and resource-intensive method. The Virtual DOM process would inherently have a hard time discerning which nodes are new, which ones were deleted and which ones merely shifted position.

Here's an example of how keys can be applied in a list rendering scenario:

let dataList = ['Apple', 'Banana', 'Cherry'];
const listItems =, index) =>
  <li key={index}>{data}</li>

In this example, array indices serve as the unique keys for the items in the list. Notice that the key is assigned within the map() function in the 'li' tag.

By augmenting the updating process with keys, React can expediently identify a node based on its key and reconcile the changes accordingly. This minimizes unnecessary component re-renders and tree traversals, thereby enhancing the overall application performance.

The Perils of Key Violation

Key violation is a term broadly used to denote the misalignment or misuse of keys. React expects keys to be consistent, unique, and attached to stable constructs. However, common oversights, such as using array indices as keys or repeating keys, potentially decrease the performance efficiency of a React application.

The usage of array indices as keys is a prevalent practice, but may lead to unpredictable issues if the list gets re-ordered or an item within the list gets deleted. Consider the following example:

let tasks = ['Task 1', 'Task 2', 'Task 3'];
const taskItems =, index) => {
  return <Task key={index} task={task} />;

In this scenario, if 'Task 1' gets deleted, 'Task 2' and 'Task 3' both slide up and occupy the indices of the preceding element. This results in key violations, whereby React performs unnecessary operations and re-renderings.

To avoid this performance pitfall, the correct approach would be to use stable unique identifiers as keys. In practice, this often translates to IDs from your data.

let tasks = [
    { id: 1, task: 'Task 1' },
    { id: 2, task: 'Task 2' },
    { id: 3, task: 'Task 3' }
const taskItems = => {
  return <Task key={} task={task.task} />;

In this approach, the unique ID serves as the key, which helps the reconciliation process, keeping unnecessary re-renders to a minimum.

In conclusion, leveraging the power of keys in React can have a profound impact on the performance of your applications. Remember, panda-wise and ensure that keys used in lists or arrays point to stable, predictable, and unique identifiers. And finally, ask yourself, are your keys following best practices, or are they merely placeholders, serving little to no use?

Introduction and Exploration of Concurrent Mode

Concurrent mode is a unique and advanced feature offered by React that enhances the smoothness of user interaction and animations. It sets a new bar for a more fluid and intuitive user interface. This capability is derived from the idea of concurrency, a fundamental concept in computer science, where multiple threads are utilized simultaneously.

To understand why concurrent mode is such a game-changer, let us elaborate on the pain points and pitfalls it attempts to address.

One of the primary causes for a sluggish user interface tends to stem from operations occupying the main thread for an excessive length of time. These operations could include the process of rendering complex component trees, making network requests, or manipulating heavy data elements.

Until now, JavaScript has primarily been a single-threaded environment, meaning tasks are executed one after the other. However, this could lead to a bottleneck scenario where long-running tasks could block off subsequent ones, stalling the responsiveness of the interface.

Concurrent mode is designed to alleviate this problem by enabling React to break the rendering work into chunks and spread it out over multiple frames, keeping the user interface responsive at all times.

Instead of blocking the main thread for a lengthy task, React in concurrent mode can start working on these heavy updates in the background. The most imperative updates can be carried out first, while lower priority updates are pushed until later.

In terms of code, when concurrent mode is used, React "suspends" the rendering of components that require additional data, allowing other components to be updated. This prevention of interface blocking is the primary strength of concurrent mode.

let rootNode = document.getElementById('root');
let concurrentRoot = ReactDOM.unstable_createRoot(rootNode);
concurrentRoot.render(<App />);

However, it's essential to ponder the impact. Concurrent Mode is not a drop-in replacement and may involve embracing a slightly different mentality towards state management and data fetching.

Two concurrent mode features that stand out are Suspense and useTransition. Suspense allows React to pause rendering a component requiring additional data, while useTransition helps mitigate abrupt changes and prevent user interface 'janking' that could occur during fast user interactions.

import React, { Suspense } from 'react';

function MyComponent() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <ComponentThatMightSuspend />

In conclusion, concurrent mode's supposedly disruptive arrival sets a new standard for user interaction and animation fluidity in applications using React. By structuring the application around these new concepts, React developers can unlock new levels of performance and user experience.

However, one might question whether the transition to Concurrent Mode is straightforward? Is adopting Suspense and useTransition a complex task, and how much would it affect the existing application architecture? Another worth-asking question is- how much performance improvement should one expect realistically with Concurrent Mode? These are in-depth questions that any senior developer should consider while experimenting and adventuring with Concurrent Mode.

Harnessing the Power of React Fiber for Enhanced Performance

React Fiber serves as a critical component of React’s reconciliation and rendering processes. It significantly refines React's underlying architecture, providing developers with additional tools to optimize their applications, and crucially, enhance performance.

React Fiber's central mechanism involves representing each React component as a Fiber node. These nodes, in turn, form a tree-like data structure known as the Fiber tree. This unique data structure implementation plays a pivotal role in enhancing React's performance.

Now, let's delve into the features offered by React Fiber and explore how they contribute to optimizing performance.

Power of React Fiber: The Non-Blocking Rendering Process

React Fiber effectively balances resource-intensive operations such as layout calculation, component generation, and user input. By introducing a non-blocking rendering process, Fiber successfully mitigates the issue of 'blocking' typically associated with JavaScript. In essence, it can concurrently manage computational heavy lifting operations alongside processing user inputs.

Such resource-intensive processes can indeed delay user interactions, leading to a less than smooth user interface. The good news is that by implementing a chunked rendering mechanism along with cooperative multitasking model, Fiber manages to distribute these processes. Specifically, heavier tasks are divided into smaller chunks, yielding flexibility in task processing. Let's look at this technique in more detail:

let nextUnitOfWork = null;

function workLoop(deadline) {
  while (nextUnitOfWork && deadline.timeRemaining() > 1) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork);


In the workLoop function above, the deadline.timeRemaining() function assigns distinct time slots for resource-intensive tasks. This aspect ensures that the reconciliation process remains 'non-blocking'. Consequently, no operation obstructs user interaction, maintaining a smooth and enjoyable user experience.

Power of React Fiber: The Suspense

Fiber brought an exceptional feature to the React playground - Suspense. It effectively manages asynchronous operations, gracefully handling transitions between different loading states with well-crafted code.

Essentially, it empowers components to 'suspend' rendering while they fetch any asynchronous dependencies. This technique is not only potent but incredibly versatile, providing developers with enhanced control over the state and rendering of their components while operations complete.

An illustrative, albeit simple, application could look like:

import { lazy, Suspense } from 'react';

const OtherComponent = lazy(() => import('./OtherComponent'));
const MyErrorBoundary = lazy(() => import('./MyErrorBoundary'));

function MyComponent() {
  return (
        <Suspense fallback={<div>Loading...</div>}>
          <OtherComponent />

In this illustration, Suspense facilitates the lazy loading of 'OtherComponent' as soon as it's available for rendering, ensuing a fallback UI during the loading phase. This transition smoothly enhances UI performance. Imagine suspending the rendering of a component with hefty processing needs, perhaps requiring an API response, until it is absolutely necessary. This can drastically reduce the CPU usage, memory footprint, and overall performance of your application.

As a natural progression, Fiber has equipped React with an array of performance, modularity, and readability features, taking the user experience and developers' satisfaction to new heights. With advances like priority scheduling, incorporated error boundaries, and the inclusion of remarkable features like Suspense, React Fiber bolsters React's standing in the world of modern web development.

For seasoned React developers, deep-diving into these concepts and scrutinizing real-world scenarios will provide valuable insights into potential strengths and weaknesses in their application's structure.

Have you come across situations where these features significantly impact performance metrics? Or are there instances where they offer minimal improvements, and the details truly make the difference? Considering these thought-provoking questions underscores Fiber's profound influence in React — further enriching your understanding of React's virtual DOM and reconciliation process.


This article provides an in-depth exploration of React's Virtual DOM and reconciliation process. It highlights the differences between the Virtual DOM and the Real DOM, emphasizing the performance advantages of using React's Virtual DOM. The article also discusses the importance of utilizing keys for performance optimization and introduces the concepts of Concurrent Mode and React Fiber.

Key takeaways from the article include:

  • React's Virtual DOM is a lightweight copy of the Real DOM that optimizes performance by batching updates and minimizing direct interaction with the DOM.
  • The reconciliation process compares the new Virtual DOM with the old Virtual DOM using a diffing algorithm to identify and update only the necessary changes in the Real DOM.
  • Using keys in lists or arrays helps React optimize the reconciliation process and minimize unnecessary component re-renders.

The article challenges readers to think about how to further optimize the performance of a React app beyond what the Virtual DOM and reconciliation provide. It also invites readers to consider scenarios where direct DOM manipulation might be more suitable and the trade-offs of choosing React's approach over direct DOM manipulation.

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