How React Works Behind the Scenes (Simple Explanation)

Posted on April 1, 2025 • 7 min read • 1,285 words

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Learn how React works behind the scenes, including the Virtual DOM, state updates, and optimizations for building fast, efficient web apps.

React has become one of the most popular JavaScript libraries for building user interfaces. Its ability to create dynamic, fast, and efficient web applications has made it a go-to tool for developers worldwide. However, many developers are often left wondering: how exactly does React work behind the scenes? Now we’ll break down React’s inner workings in simple terms, so you can better understand its magic.

1. Introduction to React

Before diving into the technical details, let’s first understand what React is and why it’s so popular.

1.1 What is React?

React is a JavaScript library used to build user interfaces, primarily for web and mobile applications. It was created by Facebook in 2011 and open-sourced in 2013. React enables developers to build reusable UI components that are easy to manage and update.

React’s key feature is the concept of the Virtual DOM, which optimizes how the browser renders changes to the user interface. This ensures fast, efficient updates without the need to reload entire pages.

1.2 Why is React So Popular?

React’s popularity can be attributed to several factors, including:

Component-based architecture: React’s component-based structure allows for code reuse and easier maintenance.
Declarative UI: React makes it easy to define how the UI should look based on the current state, without worrying about how to update the UI manually.
React’s Ecosystem: React’s vast ecosystem of libraries and tools helps developers get the job done faster and more efficiently.

In the next sections, we’ll explore how React manages to deliver this high performance and user-friendly experience.

2. React’s Core Concepts

Before understanding how React works under the hood, it’s important to grasp a few core concepts:

2.1 Components

In React, everything is a component. A component is a reusable piece of code that defines how a part of the UI should look and behave. Components can be as simple as a button or as complex as an entire page of an app. React allows you to break down your application into smaller, self-contained components, which makes development and maintenance much easier.

There are two types of components in React:

Functional Components: These are simple functions that accept props (input) and return JSX (UI elements).
Class Components: These are ES6 classes that extend React.Component and have additional features like state management.

2.2 JSX (JavaScript XML)

JSX is a syntax extension for JavaScript that looks similar to HTML but allows you to write HTML-like code inside JavaScript. React uses JSX to describe what the UI should look like. Even though JSX is not mandatory, it’s the most common way to write React components because it’s more readable and intuitive.

2.3 Virtual DOM

The Virtual DOM (VDOM) is the key concept that powers React’s efficiency. The VDOM is a lightweight in-memory representation of the real DOM. When changes are made to the app’s state, React updates the Virtual DOM first, calculates the minimal set of changes required, and then updates the actual DOM.

This approach allows React to avoid direct manipulation of the DOM, which can be slow and inefficient.

2.4 State and Props

React components can have state and props:

State: Represents the data that a component can manage and update. Changes in state trigger re-renders of the component.
Props: Short for “properties,” props are read-only inputs passed into a component from its parent. Unlike state, props cannot be changed by the component that receives them.

3. React’s Rendering Process: The Lifecycle

To understand how React works behind the scenes, we need to dive deeper into the rendering process. React follows a component lifecycle that controls how components are created, updated, and destroyed. Let’s explore how this process works step-by-step.

3.1 Initial Rendering

When a React app first loads, it needs to render the initial UI. Here’s how the process works:

Component Creation: React creates the root component (usually App) and other components defined in the code.
Rendering JSX: React processes the JSX code and converts it into JavaScript objects (React elements).
Building the Virtual DOM: React generates the Virtual DOM based on the JSX code. The Virtual DOM is a lightweight copy of the actual DOM, which React uses to track changes and optimize rendering.
Commit to the Real DOM: After rendering the Virtual DOM, React compares it to the real DOM and only updates the parts of the real DOM that have changed.

3.2 State Changes and Re-renders

Whenever the state of a component changes, React triggers a re-rendering process. Here’s how React handles state updates:

State Update: When the state of a component changes, React schedules a re-render for that component.
Virtual DOM Update: React updates the Virtual DOM to reflect the new state.
Diffing Algorithm: React uses an efficient algorithm (called the Reconciliation algorithm) to compare the old Virtual DOM with the new one. It then calculates the minimum number of changes needed to update the real DOM.
Commit Phase: After React calculates the differences, it applies the changes to the actual DOM, ensuring that only the necessary updates are made.

This process ensures that React applications stay performant, even when dealing with frequent state updates.

4. The Diffing Algorithm: How React Optimizes DOM Updates

The Diffing algorithm is one of the key innovations that make React so fast. When state changes, React compares the previous Virtual DOM with the new one to figure out what changed. This comparison process is called Reconciliation.

Here’s how it works:

4.1 Key Principles of Diffing

React assumes that elements of the same type will produce similar trees. For example, if a component renders a <div>, React expects that future <div> elements will have similar characteristics. This allows React to optimize its comparisons.

4.2 Efficient Updates

Once React detects a change, it needs to determine which parts of the UI have changed. React does this efficiently by:

Reusing Components: If the component structure hasn’t changed, React can reuse the same DOM elements and update only the necessary parts.
Minimal DOM Updates: React calculates the minimal number of changes needed to update the DOM, reducing the time spent on expensive DOM operations.

This approach drastically reduces the amount of work React needs to do, leading to better performance.

5. The Role of React’s Hooks

In recent versions of React, Hooks have become an essential feature. Hooks allow developers to manage state and side effects in functional components without the need for class components.

5.1 useState and useEffect

useState: This hook lets you add state to functional components. When the state changes, the component re-renders automatically, similar to how state works in class components.
useEffect: This hook lets you perform side effects in functional components, such as fetching data, setting up subscriptions, or manually updating the DOM.

Hooks allow for cleaner and more concise code, making React applications easier to maintain.

6. React’s Optimizations: Why It’s So Fast

React’s performance comes from several optimizations that help minimize unnecessary re-renders and DOM manipulations:

6.1 Batching State Updates

React batches multiple state updates into a single render, which improves performance by reducing the number of re-renders.

6.2 Memoization with React.memo

For functional components, React provides a higher-order component called React.memo. This feature memoizes the component, preventing unnecessary re-renders when props haven’t changed.

6.3 Lazy Loading and Code Splitting

React allows for lazy loading of components and code splitting, which means only the code needed for the current view is loaded. This reduces the initial load time and improves the app’s performance.

7. Conclusion

We’ve explored how React works behind the scenes, from components and JSX to state updates and the Virtual DOM. We’ve also discussed React’s efficient rendering process, the Diffing algorithm, and optimizations like hooks and memoization. Understanding these internal mechanisms can help you write more efficient and effective React applications.

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