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Understanding the React Engine: How It Works Internally


React is a popular JavaScript library for building user interfaces, known for its efficient and declarative approach to rendering UI components. At its core, React relies on a virtual DOM and a mechanism called the "React engine" to efficiently update and render components in web applications. In this blog post, we'll dive into the internals of the React engine to understand how it works.

What is the React Engine?

The term "React engine" is not an official React concept but is often used to refer to the internal workings of React that drive its component rendering and updates. At its heart, the React engine consists of three key processes: Reconciliation, Rendering, and Diffing.


Reconciliation is the process of determining which parts of the virtual DOM need to be updated based on changes in the application's state. React accomplishes this through a process known as the "Reconciliation algorithm." When changes occur in a component's state or props, React creates a new virtual DOM representation of the component tree.

1.Element Comparison : React compares the new virtual DOM tree with the previous one to identify the differences. It uses a unique "key" attribute to optimize this process, ensuring that elements are compared efficiently.

2. Component Lifecycle Methods: During reconciliation, React invokes various component lifecycle methods such as `componentWillUnmount`, `componentDidMount`, and `componentDidUpdate` to handle component state changes and side-effects.

3. Virtual DOM Diffing: React determines the minimum number of changes required to update the actual DOM. It does this by performing a "diffing" operation on the virtual DOM trees, identifying what has changed.


Rendering is the process of updating the actual DOM based on the changes identified during reconciliation. React uses a process called "re-rendering" to efficiently update the real DOM.

1. Batching : React batches multiple updates and applies them in a single pass, reducing the number of DOM manipulations. This process is crucial for performance optimization.

2. Fiber Architecture: React introduced the Fiber architecture to enable asynchronous rendering. This allows React to prioritize updates and maintain a responsive user interface even when performing computationally intensive tasks.


Diffing is the heart of the React engine, where the real magic happens. During the diffing process, React efficiently calculates the minimal set of changes needed to update the actual DOM. It uses a tree-differencing algorithm to achieve this.

1. Virtual DOM Tree Comparison: React compares the new virtual DOM tree with the previous one, identifying nodes that have changed, been added, or been removed.

2Reconciliation Strategy: React employs a reconciliation strategy to decide whether to update, replace, or re-order elements in the real DOM. This strategy optimizes performance by minimizing DOM manipulations.

3. Virtual DOM Patches: React generates a series of "patches" or instructions that describe the changes required to bring the actual DOM in sync with the virtual DOM.


The React engine is at the core of what makes React a powerful and efficient library for building user interfaces. It combines the processes of reconciliation, rendering, and diffing to update the DOM with minimal changes, leading to better performance and a smoother user experience.

Understanding how the React engine works internally can help developers write more efficient React applications. By leveraging React's built-in optimizations and following best practices, you can create web applications that are both responsive and maintainable. As React continues to evolve, staying informed about its internal workings will be essential for keeping your skills up-to-date.

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