Understanding React Fiber

React Fiber is a highly efficient and concurrent reconciliation engine Facebook (meta) introduced in 2017. It serves as the foundation of the React library. It enables developers to build responsive and interactive user interfaces (UIs) for web applications.

The designers of React Fiber addressed performance bottlenecks and improved the user experience by introducing asynchronous rendering and efficient prioritization of updates. In this article, we will explore the concept of React Fiber. Its key features, and the benefits it brings to the development of modern web applications.

The Need for React Fiber

Before the introduction of React Fiber, React relied on a process known as “stack reconciliation” to update the UI. In stack reconciliation, React traversed the entire component tree in a single go, which could lead to performance issues when dealing with large and complex UIs.

As the web platform evolved and application complexity grew, it became apparent that React needed a more efficient way to handle UI updates and ensure a smooth user experience.

Introducing React Fiber

React Fiber represents a complete reimplementation of the React reconciliation algorithm, designed to enable more granular control over the rendering process.

It is named “Fiber” because it uses a new data structure called “fiber” to represent components and their relationships in the render tree.

What does the “fiber” data structure look like?

Tile mosaic structure

The fiber data structure is a key component of React Fiber and represents individual components and their relationships in the render tree. It serves as a lightweight and mutable representation of a component, enabling React to manage and update the UI efficiently.

A fiber object contains various properties that capture essential information about a component and its state. A typical fiber consists of the following key properties:

  1. Type: This property identifies the component type, such as a functional component, class component, or host component (e.g., div, span).
  2. Key: In an array of children, the key assigns a unique identifier to a component. It helps React optimize updates and track components more effectively during reconciliation.
  3. Props: The props property holds the component’s properties or attributes passed by its parent component. It includes data like event handlers, styles, and other relevant information.
  4. StateNode: This property refers to the instance of the component class or the DOM node associated with a host component. It represents the actual instance of the component that renders or undergoes updates.
  5. Return: The return property points to the parent fiber in the tree. It allows React to navigate the fiber tree and efficiently traverse the component hierarchy during reconciliation.
  6. Sibling: The sibling property refers to the next fiber within the same level of the component tree. It helps React traverse and reconcile components efficiently, avoiding unnecessary traversals of unrelated components.
  7. Child: The child property points to the first child fiber of a component. It enables React to traverse and reconcile the component’s children efficiently.
  8. Alternate: The alternate property points to the previous fiber representing the same component. During the reconciliation process, it compares the previous and current states of the component, enabling React to determine the necessary updates based on the changes detected.

How does this structure help?

These properties allow React Fiber to efficiently traverse, update, and reconcile the component tree during rendering. By representing components as lightweight and mutable fibers, React can optimize the rendering pipeline, prioritize updates, and achieve better performance in web applications.

Key Features of React Fiber

  1. Asynchronous Rendering: React Fiber introduces asynchronous rendering, enabling interruptions and resumptions of the rendering process for a more responsive user interface. React can work on rendering components incrementally, prioritizing high-priority updates and deferring lower-priority updates to avoid blocking the main thread.
  2. Prioritization and Time Slicing: React Fiber implements a scheduling algorithm that allows developers to assign priority levels to different updates. This prioritization ensures that critical updates, such as user interactions or animations, are processed with higher priority, resulting in a more fluid and responsive UI. Time slicing further enhances this feature by breaking the rendering work into smaller units, preventing the UI from becoming unresponsive.
  3. Incremental Rendering: React Fiber introduces the ability to split the rendering work into multiple “chunks” or “frames.” It allows the browser to process these smaller work units in between frames, thus minimizing the impact on the user interface. This approach contributes to smoother animations, faster updates, and a more interactive experience for the end user.

An Example of React Fiber

One good example demonstrating React Fibre’s benefits is handling large lists or tables with dynamic data. Rendering many items in a list or table can be computationally expensive and may lead to performance issues, especially when updates occur frequently.

React Fiber uses asynchronous rendering and time slicing to optimize the rendering process. Imagine a situation where we must present a table format to showcase a vast list of user data.

In a traditional rendering approach, updating the entire table when a single user’s data changes could be inefficient. React Fiber addresses this issue by breaking down the rendering work into smaller chunks and utilizing time slicing. Here’s how it works:

  1. Initial Rendering: When the table component is mounted, React Fiber divides the rendering work into smaller units, or fibers, and assigns priority levels based on their importance. The initial rendering will prioritize the visible portion of the table to ensure a quick and responsive user interface.
  2. Time Slicing: React Fiber then allocates time slots to process these smaller work units. It interleaves the rendering work with browser rendering cycles, ensuring the UI remains responsive and doesn’t freeze.
  3. Incremental Updates: A user’s data is updated, and the table needs to reflect this change. React Fiber allows the update to be prioritized based on its importance, such as highlighting the updated row or changing the relevant cell’s value. Only the affected portions of the table are updated, while the rest of the UI remains unaffected.
  4. Asynchronous Rendering: React Fiber’s asynchronous rendering ensures that the UI remains responsive even while processing updates. It can interrupt the rendering process to handle higher-priority user interactions or events, such as scrolling or input, without blocking the main thread.

Benefits of React Fiber

  1. Improved Performance: React Fibre significantly improves React applications’ performance by adopting asynchronous rendering and prioritization. It minimizes the time spent on rendering and maximizes the time available for handling user interactions, resulting in a smoother and more responsive user experience.
  2. Better User Experience: The ability to prioritize updates and time slice rendering enables developers to create more interactive and engaging web applications. Users can experience a seamless and responsive interface as critical interactions are promptly reflected in the UI. React Fiber’s design encompasses future-proof architecture. Its flexible and extensible architecture provides a strong foundation for further optimizations and features. It allows React to adapt to evolving web standards and browser technology advancements, ensuring React-based applications stay performant and up-to-date.


React Fiber revolutionizes how React handles UI updates, introducing asynchronous rendering, prioritization, and time slicing. These features optimize performance, ensure a smooth user experience, and lay the groundwork for future advancements. React Fiber empowers developers to create highly performant web applications, delivering fast and interactive user interfaces. By understanding and leveraging the capabilities of React Fiber, developers can take full advantage of React’s potential and build modern, responsive, and engaging web applications.

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