Exploring React Fiber Architecture

  • by Haozheng Li

In the React community, the Fiber architecture is undoubtedly one of the biggest technological innovations in recent years. This blog post will delve into various aspects of the Fiber architecture, from its concept and necessity to the specific changes and features it brings, as well as its implementation principles and impact on component lifecycles. Through this article, I aim to provide developers with a comprehensive understanding of why the Fiber architecture is a key advancement in modern web application development.

1. What is Fiber Architecture?

The Fiber architecture is a major update to React that has completely transformed the internal workings of React. "Fiber," which means "fiber" in English, symbolizes its design philosophy—like a fiber, the Fiber architecture aims to control and optimize React's rendering process through finer-grained task units.

In computer science, a Fiber is a lightweight cooperative multitasking structure that allows code to execute tasks collaboratively, rather than through traditional preemptive multithreading. React's Fiber architecture has adopted this concept by breaking down rendering tasks into small, pausable, and resumable units (known as "Fibers"), allowing React's rendering engine to pause operations when necessary and later continue from where it stopped. Each Fiber node represents a unit of work or part of a component.

2. Why is Fiber Architecture Needed?

To deeply understand why React introduced the Fiber architecture, we first need to explore the browser's event loop mechanism, especially how it handles JavaScript execution and UI rendering. This will help us comprehend why the traditional React rendering mechanism encounters performance bottlenecks in complex applications.

The Browser's Event Loop and Rendering Cycle

One of the core functions of a browser is to continuously render and update the user interface, achieved through a mechanism known as the "event loop." The main tasks of the event loop include handling user input (such as mouse clicks and keyboard events), executing JavaScript code, performing style calculations, layout (Reflow), and painting (Paint).

  1. JavaScript Execution Cycle: When you run JavaScript in a browser, it operates in a single-threaded environment, meaning only one task can be executed at any given time. The execution of JavaScript can affect the DOM, requiring the browser to recalculate styles and layout before proceeding with other rendering steps.
  2. Rendering Cycle: This involves recalculating styles, determining the layout of DOM elements, and the final pixel painting. This process needs to be highly optimized, as any delays can directly impact the user experience. Ideally, browsers aim to complete a render once every refresh cycle of the display (usually 60 times per second, or every 16.67 milliseconds) to maintain a smooth visual effect.

3. Changes and Solutions Introduced by the Fiber Architecture

The introduction of the Fiber architecture brought about a series of crucial changes to React, not only optimizing the framework's internal mechanisms but also directly addressing longstanding issues with performance and responsiveness. Here are the major changes brought by the Fiber architecture and the specific problems they solve:

Interruptible Rendering

In the old React architecture, once rendering of a component began, it had to be carried through to completion, processing the entire component tree in one go. This synchronous rendering approach worked well for smaller component counts but led to significant UI freezes in large applications, especially during complex updates or with deep component trees.

The Fiber architecture has transformed this by introducing an interruptible rendering process. React can now break down the rendering work into smaller units of tasks, each of which can be paused and resumed. This flexibility allows React to adjust its execution strategy based on the current environment and task priority. For example, during user interactions, React can pause ongoing rendering tasks to prioritize responding to the user, significantly enhancing the application's responsiveness and user experience.

Task Priority Management

The Fiber architecture includes a system for managing task priorities, allowing different types of updates to be assigned various levels of importance. This is managed by React's scheduler, which dynamically adjusts the priority of tasks based on their nature, such as animations, user input responses, and background data synchronization.

This prioritization system enables React to execute tasks that have the most significant impact on user experience first, while less urgent tasks can be deferred. For instance, input responsiveness and animation rendering typically receive higher priority because they directly affect user perception; meanwhile, tasks like data updates and logging may be performed during idle times.

Improved Error Handling and Boundary Management

With the Fiber architecture, React has introduced a more robust error handling mechanism. The concept of Error Boundaries allows developers to better capture and manage errors that occur within the component tree, preventing the entire application from crashing. This is especially crucial for building large, stable applications, as it ensures that issues in individual components do not compromise the overall stability of the application.

Support for Concurrent Mode

The Fiber architecture lays the foundation for React's Concurrent Mode. Concurrent Mode allows React to prepare multiple versions of the UI in the background, which can quickly adapt to different user inputs without the need to rebuild the entire tree. This mode is particularly valuable in applications where data changes frequently, as it ensures that the UI remains consistent with the latest state while maintaining high performance and a smooth user experience.

4. Incremental Rendering in Fiber Architecture

Incremental rendering is a core feature within the Fiber architecture, allowing React's rendering process to be broken down into multiple small, independent work units. Each unit can be independently scheduled, interrupted, and resumed. This mechanism significantly enhances the responsiveness and performance of React applications, especially during large-scale updates.

Concept of Work Units

In the Fiber architecture, each React element is internally transformed into a work unit known as a Fiber. Each Fiber node represents a portion of a component instance's work, such as lifecycle method invocations, state updates, and virtual DOM reconciliation. The structure of a Fiber node is designed to remember its children, parent, and sibling nodes, supporting a flexible traversal process that allows tasks to be interrupted and resumed at any node.

Here is a simplified and abstract example demonstrating how to create a Fiber node for a component and simulate the creation of Fiber nodes during an update process:

function createFiberNode(component, props) {
    return {
        type: component.type,
        props: props,
        state: component.state,
        parent: null,
        child: null,
        sibling: null,
        alternate: null, // Links to the old Fiber node for comparison and reuse

function updateComponent(component, newProps) {
    const oldFiber = component.fiber;
    const newFiber = createFiberNode(component, newProps);
    newFiber.alternate = oldFiber; // Keep a reference to the old node
    reconcileChildren(oldFiber, newFiber); // Compare and update children
    return newFiber;

// Simulate component and properties
const MyComponent = { type: 'MyComponent', state: {} };
const initialProps = { children: 'Hello World' };
const updatedProps = { children: 'Updated Hello World' };

// Create and update Fiber nodes
const initialFiber = createFiberNode(MyComponent, initialProps);
MyComponent.fiber = initialFiber; // Associate the Fiber node with the component
const updatedFiber = updateComponent(MyComponent, updatedProps);

Interruptible Task Execution

React uses a scheduling algorithm to determine which Fiber nodes should be prioritized. As React begins the rendering or updating process, it builds or updates the Fiber tree incrementally. This tree-building process involves traversing Fiber nodes and executing the work each node represents, such as invoking render methods and reconciling new and old virtual DOMs.

Crucially, this process is no longer monolithic. At any stage, if the main thread needs to handle higher-priority tasks (such as user input), React can suspend the current work. This decision is made using React's shouldYield method, which checks the browser's frame rate and task execution time to determine whether to interrupt the current work.

Task Priority and Scheduling

Task scheduling within the Fiber architecture is priority-based. React defines multiple levels of priority, such as synchronous execution, task blocking, and normal priority. Each Fiber node update can be assigned a priority based on its type and context. The React scheduler uses these priorities to decide which tasks should be executed immediately and which can be deferred.

Using requestIdleCallback and requestAnimationFrame, the React scheduler can finely control the timing of task execution. requestAnimationFrame ensures that animations or visual updates are executed before the next frame, while requestIdleCallback is used to perform low-priority tasks when the main thread is idle.

Recovery from Interruptions

When React decides to interrupt the processing of a Fiber node in response to more urgent tasks, it remembers the current execution context. This includes both the Fiber nodes that have been processed and those that have not yet been completed. Once the high-priority task is completed, React can resume work from where it was interrupted, continuing the previous rendering process.

This recovery process is seamless because each Fiber node contains enough information to restart or continue its task, regardless of the stage it was in before the interruption. This ensures that even with frequent interruptions, the overall state of the application remains consistent.

While the implementation of interruptions in React's internal Fiber architecture is complex, a simplified example can illustrate the concept:

function shouldYield() {
    // Simplified example; in reality, React uses browser APIs and time slicing calculations
    return window.performance.now() % 5 === 0; // Assume an interruption is considered every 5ms

function performWork(fiber) {
    console.log(`Processing fiber: ${fiber.props.children}`);
    // Assume some rendering logic here
    if (fiber.child) {
        return fiber.child; // Return the next Fiber node to process
    // More logic could be added here to simulate a complete traversal process
    return null;

let nextUnitOfWork = initialFiber; // Start from the root node

function workLoop(deadline)

    while (nextUnitOfWork && !shouldYield()) {
        nextUnitOfWork = performWork(nextUnitOfWork); // Perform work and move to the next unit
    if (nextUnitOfWork) {
        requestIdleCallback(workLoop); // If there is still work to do, continue scheduling

// Simulate the browser's idle callback

This approach showcases how Fiber's incremental and interruptible rendering enhances React's performance and user experience, particularly in complex applications.

5. Error Handling and Boundary Management in Fiber Architecture

In React's Fiber architecture, error handling and boundary management are integral components of the design, especially in asynchronous rendering environments. They provide a stable and predictable user experience. This section explores how Fiber addresses errors within the component tree and prevents entire application crashes by introducing the concept of Error Boundaries.

Concept of Error Boundaries

Error Boundaries are React components that capture JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of allowing the whole tree to crash. This functionality is achieved through the lifecycle method componentDidCatch, which has been specially optimized within the Fiber architecture.

class ErrorBoundary extends React.Component {
    constructor(props) {
        this.state = { hasError: false };

    static getDerivedStateFromError(error) {
        // Update state so the next render will show the fallback UI.
        return { hasError: true };

    componentDidCatch(error, info) {
        // You could also log error information to an error reporting service
        logErrorToMyService(error, info);

    render() {
        if (this.state.hasError) {
            // You can render any custom fallback UI
            return <h1>Something went wrong.</h1>;

        return this.props.children;

In the code above, the ErrorBoundary component uses getDerivedStateFromError to set the fallback state, capturing errors and their details with componentDidCatch, and optionally sending error information to an error reporting service.

Capturing and Propagating Errors

In the Fiber architecture, each Fiber node may throw errors during execution. React's scheduler is responsible for managing these errors, ensuring they are captured by the appropriate Error Boundaries. If a Fiber node throws an error, React will traverse up from that node to its parent nodes until it finds the nearest Error Boundary.

This error capture mechanism relies on the tree-like structure of Fiber nodes, allowing React to efficiently locate and mark parts of the component tree that have encountered errors, and to replace them with a fallback UI.

function simulateErrorInComponent() {
    throw new Error("Simulated error");

function ComponentThatMayError({ hasError }) {
    if (hasError) {
    return <div>Component is working fine!</div>;

function App() {
    return (
            <ComponentThatMayError hasError={true} />
            <ComponentThatMayError hasError={false} />

In this example, the ComponentThatMayError decides whether to throw an error based on the hasError prop. The ErrorBoundary ensures that even if one component fails, other components can continue to render normally.

Error Handling in Concurrent Mode

Another key advantage of the Fiber architecture is how it handles errors in Concurrent Mode. In this mode, React can "pause" an update, and if the update throws an error, React may decide to discard the entire update or just part of it, without affecting the user's current interface.

This capability significantly enhances the robustness of the application, allowing developers to experiment with and implement more dynamic UI update strategies without worrying about crashing the entire application.

Through these mechanisms, the Fiber architecture offers a more powerful and flexible way to handle runtime errors, enhancing the stability and user experience of React applications.

6. Priority in the Fiber Architecture

In the Fiber architecture, task priority is a core concept that enables React to schedule and manage rendering work more intelligently. The prioritization system ensures that tasks critical to user experience are processed more quickly, while less urgent tasks can be deferred. This flexible scheduling is crucial for enhancing the responsiveness and performance of applications.

Types and Allocation of Priorities

React defines multiple levels of priority to finely control the order in which updates are processed. These priorities range from urgent to deferrable:

  • Immediate Priority: For updates that need to be executed immediately, such as keyboard inputs and click events, React assigns the highest priority. This ensures that the application responds to user inputs instantly, enhancing the smoothness of interactions.

  • User Blocking Priority: Used for interactions that require a quick response but can be slightly delayed, such as drag feedback or animations, these tasks are typically completed within tens of milliseconds.

  • Normal Priority: Most updates are assigned this level, such as rendering a page after data retrieval. These tasks are important but do not need to be completed immediately.

  • Low Priority: For tasks that have a minimal impact on user experience, such as logging or data storage, a lower priority can be assigned.

  • Idle Priority: When there are no more critical tasks to handle, these tasks can be performed, such as preloading certain resources or carrying out non-urgent background work.

Task Scheduling and Execution

How does the task scheduler in the Fiber architecture work with these priorities? It relies on an internal scheduling algorithm that determines which tasks should be executed and which can be paused or deferred. The scheduler uses browser APIs such as requestIdleCallback and requestAnimationFrame to assist in scheduling tasks, making more efficient use of the browser's idle time.

  • requestAnimationFrame (rAF): React uses rAF for visual updates, such as animations and page repaints. This ensures that UI updates occur at the start of each frame, minimizing visual jitter and delays.

  • requestIdleCallback: This API allows React to perform low-priority tasks when the main thread is idle, without impacting crucial user interactions.

Through this flexible priority management and meticulous task scheduling, the Fiber architecture significantly enhances the performance of React applications, particularly when dealing with large-scale data updates and complex interactions. This strategy ensures that applications can respond quickly to user actions while effectively utilizing system resources to prevent browser stalling and crashes, thereby providing a smooth and pleasant user experience.

7. Impact of Fiber Architecture on Lifecycle Methods

The introduction of the Fiber architecture has significantly impacted the lifecycle methods of React components. These changes aim to enhance efficiency and safety while better accommodating concurrent rendering. Let's explore in detail how the Fiber architecture has altered lifecycle methods and what these changes mean for developers.

Introduction of New Lifecycle Methods

To better support asynchronous rendering and error handling, React introduced new lifecycle methods and gradually deprecated some old ones. These new methods provide more control over the rendering process and help developers avoid common mistakes.

  • getDerivedStateFromProps: This method replaced componentWillReceiveProps and became a static method. It is called before a component receives new props, allowing state updates based on changes in props, which helps prevent issues caused by direct manipulation of state.

  • getSnapshotBeforeUpdate: Called right before the latest DOM changes are committed, this method allows capturing information (such as scroll position) before the component updates. The captured information can then be used in componentDidUpdate for additional operations.

Deprecated Lifecycle Methods

With the introduction of new methods, several old lifecycle methods have been deprecated mainly because they could introduce bugs in an asynchronous rendering environment.

  • componentWillMount: This method was called before rendering and could be invoked multiple times, which is unpredictable in asynchronous rendering modes. It has been replaced by componentDidMount.

  • componentWillReceiveProps: This method could lead to extra renderings or be called multiple times during the rendering process. It has been replaced by getDerivedStateFromProps, which offers a safer way to update state based on changes in props.

  • componentWillUpdate: In the new rendering process, using getSnapshotBeforeUpdate and componentDidUpdate allows for safer management of state changes before and after updates.

Safe Use of Lifecycle Methods

In the Fiber architecture, considering that the execution of lifecycle methods can be interrupted and restarted, developers need to ensure the purity and reentrancy safety of these methods. This means avoiding side effects (such as API calls and data subscriptions) within lifecycle methods, unless these actions are placed in componentDidMount or componentDidUpdate.


By introducing new lifecycle methods and phasing out those unsuited for asynchronous rendering, the Fiber architecture significantly improves the rendering efficiency and reliability of components. These changes require developers to update their understanding of component lifecycles and adapt to new best practices, thereby fully leveraging React's performance improvements to write more robust and efficient code. In this way, React ensures its leadership in modern web application development, providing a more powerful and flexible toolset to meet the growing needs of users and businesses.

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