[Advanced React] What is React Fiber? Understanding Its Architecture and How It Works

In the fast-paced world of web development, React has become an undeniable force, helping developers build complex user interfaces (UIs) efficiently. But behind the simplicity of components and hooks lies a powerful engine—a digital "heart" that beats constantly to keep everything smooth, fast, and responsive. That engine is React Fiber.

If you've ever wondered how modern React apps can handle complex animations, async data loading, and instant user feedback without "jank," the answer lies in this revolutionary architecture. Let's take a deep dive into React, decode the Fiber architecture, and see why it's a true game-changer.

The Problem of the Past: Stack Reconciler

Before Fiber, React used an algorithm called the "Stack Reconciler." Imagine it as a chef who can only do one thing at a time. When a UI update was needed (e.g., a button click), the old reconciler would start the reconciliation process—comparing the new Virtual DOM tree to the old one to find changes to apply to the real DOM.

The biggest problem: it was synchronous and could not be interrupted. Once reconciliation started, it had to finish before anything else could happen. If your component tree was large and complex, this could take tens or even hundreds of milliseconds. During that time, the browser's main thread was completely blocked.

The result?

• Frozen UI: Users couldn't interact, inputs wouldn't respond, and worst of all, animations would stutter and lag, creating a terrible user experience.
• Wasted resources: There was no way to prioritize important tasks. A small user click could be forced to "wait in line" behind a huge, non-urgent render task.

React needed a new heart—one that could beat to the user's rhythm, pause for urgent work, and resume unfinished business later. React Fiber was born to solve this problem.

What is React Fiber? A Revolution in Reconciliation

At its core, React Fiber is a complete reimplementation of React's core reconciliation algorithm. It's not a feature you import into your code, but a foundational architecture—a fundamental change in how React thinks and works.

Fiber's main goal is to let React pause, abort, or restart rendering work flexibly. Instead of treating UI updates as a single, indivisible block, Fiber breaks them into smaller "units of work." The main thread can process a few units, then pause to check for more important tasks (like user input), handle them, and then resume unfinished work.

This is called asynchronous rendering or concurrent rendering.

The Heart of Fiber: The "Fiber" Data Structure

To make this magic possible, React introduced a new data structure called a fiber node. Each fiber node is a JavaScript object containing information about a component, a DOM element, or a piece of UI. It's not just a copy of a component in the Virtual DOM tree—it is a unit of work.

A fiber node contains important info like:

• type and key: Just like in the React elements you write.
• child and sibling: Pointers to the first child fiber node and the next sibling, forming a linked list.
• return: Pointer back to the parent fiber node.
• pendingProps and memoizedProps: New props and the props used in the previous render.
• stateNode: Pointer to the component instance or DOM element.

By using child, sibling, and return pointers, React turns the hierarchical component tree into a flat linked list. This lets React traverse the tree and process each node (unit of work) independently, without deep recursion like the old reconciler.

How It Works: Two Trees, Two Phases

React Fiber works with a smart mechanism using two trees and a two-phase render process.

1. The current and workInProgress Trees

At any time, React maintains two fiber trees:

• The current tree: Represents the UI currently displayed on the screen. It's the source of truth.
• The workInProgress tree: When an update is requested, React doesn't change the current tree directly. Instead, it creates a copy called the workInProgress tree. All calculations, comparisons, and changes happen here.

Working on a "draft" (workInProgress) ensures the current tree and the real UI aren't affected by ongoing rendering—even if it's paused or interrupted.

2. The Two Phases of Rendering

UI updates are split into two clear phases:

Phase 1: Render/Reconciliation (Asynchronous & Interruptible)

This is where React builds the workInProgress tree. Starting from the root, it traverses each fiber node and:

• Calls render() for class components or executes function components.
• Compares the result to the old child fiber node.
• Determines what changes are needed (add, update, delete).

This phase is completely safe to pause. Since it's all in memory and doesn't touch the real DOM, React can stop and resume at any time without side effects. React can run this phase in small chunks, yielding control to the browser after each chunk to handle high-priority events.

Phase 2: Commit (Synchronous & Non-interruptible)

Once the workInProgress tree is ready, React enters the Commit phase. Here, React:

• Walks through the finished workInProgress tree and applies all calculated changes to the real DOM in a single batch.
• Runs lifecycle methods like componentDidMount, componentDidUpdate, and hooks like useEffect, useLayoutEffect.

This phase must be synchronous and cannot be interrupted to ensure DOM consistency and avoid half-finished UI states.

After Commit, the workInProgress tree becomes the new current tree, and the cycle is ready for the next update.

The Big Benefits: More Than Just Performance

Fiber's architecture isn't just a performance boost—it opens the door to powerful new features in React:

• Smooth user experience: By prioritizing important tasks (like user input) and breaking up large render tasks, Fiber nearly eliminates jank and lag, even in the most complex apps.
• Concurrency: This is the core capability. React can work on multiple state updates at once, with different priority levels.
• React Suspense: Lets components "wait" for async tasks (like data or code loading) before rendering, while showing a fallback UI. This wouldn't be possible without Fiber's ability to pause rendering.
• useTransition and useDeferredValue: These hooks let developers mark state updates as "non-urgent," so React can delay rendering them to prioritize more important interactions.

What Does This Mean for Developers?

For most React developers, Fiber works silently behind the scenes. You don't need to change how you write components or manage state day-to-day.

However, understanding Fiber helps you:

1. Write more optimal code: You'll know why keeping render tasks short matters, and how to use new tools like useTransition to improve app responsiveness.
2. Debug more effectively: When you hit performance issues, you'll have the background to understand what's happening and use tools like React Profiler more effectively.
3. Leverage modern features: You'll grasp the essence of Suspense, Concurrent Mode, and future React features—all built on Fiber.

Conclusion: React Fiber is a Complete Overhaul

React Fiber isn't just an update. It's a complete overhaul—a new brain and heart that lets React meet the challenges of modern web apps.

By breaking up work, setting priorities, and working asynchronously, Fiber has turned React from a library that "works hard" into one that "works smart." No matter how complex your app gets, Fiber ensures the user experience always comes first—fast, smooth, and never interrupted.