Software Developer @ThoughtWorks.
Agile methodologies devotee.
JavaScript lover.

What is React Fiber ?

During the last months, the interest of the community to the upcoming version of React has grown rapidly, especially after the awesome job Lin Clark did to explain how React Fiber works with cartoons at React Conf 2017.

So, what is React Fiber in a nutshell?

React Fiber is a reimplementation of the React reconciler.

It’s a ground-up rewrite of the React 15 and older versions reconciliation algorithm which now has been retroactively called React Stack because it is based on recursion.

DISCLAIMER: The following are personal notes and deductions after exploring the new React Fiber reconciler codebase. Some of the content can be wrong or not up to date, please open a pull request if you notice any mistake. 🙏

What is the reconciler ?

When React first came out, the most revolutionary feature was the Virtual DOM because it makes writing applications a lot easier. Instead of telling the browser exactly what it needs to do in order to update your UI, you would tell React how the next state of your application should look like and it would take care of everything in between.

The reconciler is the part of React which contains the algorithm used to diff one tree with another to determine which parts need to be changed.

In the last years React has been refactored so that reconciliation and rendering are separate phases. The reconciler does the work of computing which parts of a tree have changed; the renderer then uses that information to actually update the rendered app. react-dom and react-native are only two of the many renderers pluggable into React.

You can find out more about the reconciliation process in the Official React Documentation.

Why rewrite the reconciler ?

Probably one of the main reasons is this historical problem we have in browsers:

The main thread is the same as the UI thread.

Rendering the page, responding to user actions, computing JS, managing network activities, and manipulating the DOM is all handled by the browsers main thread. Although today some of these things can be moved to another thread safely and with relative ease using Workers, only the main thread can change the DOM.

In React applications, when the state changes or the props update, the render() function creates a new tree of React Elements and then React runs the reconciliation algorithm to figure out how to efficiently update the UI to match the new tree.

The React Stack reconciler always processes the component tree synchronously in a single pass. This prevents the main thread from doing potential urgent work until the recursive process has finished. If this computation runs while the user happens to type in a text input, your app can become unresponsive, resulting in choppy frame rates and laggy input.

The new Fiber reconciler’s main goal is to split interruptible work into chunks with the ability to assign priority to different types of updates so that the main thread could decide to pause the diff algorithm, potentially do some more urgent work, and continue from where it left off at a later time.

As mentioned in the scheduling section of the React Design Principles Documentation Fiber will enable the following functionalities:

If something is offscreen, we can delay any logic related to it. If data is arriving faster than the frame rate, we can coalesce and batch updates. We can prioritize work coming from user interactions (such as an animation caused by a button click) over less important background work (such as rendering new content just loaded from the network) to avoid dropping frames.

The triangles demo

To better understand the benefits React Fiber can provide to our React applications, Sebastian Markbåge, who is considered the “tech lead” of the React Core Team, built a useful fractals example. Check out the following video:

Source code of the Triangles Demo can be found in the Official Github React Repository.

As you can see from the video, there are 2 different kinds of updates going on in this demo.

One that make the triangles narrow and wide which has to happen every 16ms (60 FPS) in order for the animation to look smooth and one that updates the numbers contained inside every single dot which happens approximately every 1000ms.

This example is perfect for analysing a situation where we want to be able to assign different priorities to different types of updates. The animation update which makes the triangles wide and narrow is more important than the numbers one. If the numbers are updated with some delay the user will probably not even notice it. On the other hand, if the animation starts to drop frames we would end up staring at a janky motion.

Let’s use this example to check out how the Fiber reconciler works and how we can split work into chunks.

A taste of React Fiber architecture

Andrew Clark’s React Fiber Architecture document is so good on explaining the idea behind Fiber implementation that I’ll just quote it here:

Fiber is a reimplementation of the stack, specialized for React components. You can think of a single fiber as a virtual stack frame.
The advantage of reimplementing the stack is that you can keep stack frames in memory and execute them however (and whenever) you want. This is crucial for accomplishing the goals we have for scheduling.

And more in detail:

A fiber is a JavaScript object that contains information about a component, its input and output. At any time, a component instance has at most two fibers that correspond to it: the current, flushed fiber, and the work-in-progress fiber.

And here is what a very simplified version of a fiber (virtual stack frame) looks like:


Digging through the properties of a fiber goes beyond the purpose of this post.
If you are interested in understanding them I strongly recommend reading the document written by Andrew Clark. You can also play around with the Fiber Debugger developed by Dan Abramov to visualise how Fiber works internally.

Animation Update Timeline

Animation update timeline from the Triangles demo.

React Fiber introduces phases in the reconciliation algorithm.

There is a first render / reconciliation phase where Fiber will build up a working-in-progress tree and figure out a list of the changes it needs to make in order to update the UI. It will not make any actual changes, though, at this stage.

This first phase is interruptible and we will see later in the example how this peculiarity becomes key in allowing high priority updates to jump ahead of low priority updates.

Then there is a commit phase where Fiber will actually make all the changes figured out in the previous phase to the DOM.

This second phase is uninterruptible because we do not want to end up with an inconsistent UI. Partial updates can be annoying to the user and Fiber avoids them.

Fiber will also call the lifecycle hooks and handle error boundaries as part of the commit phase.

The triangles demo without time slicing (janky animation)

Let’s try to figure out from the following timeline what causes the janky animation.

Triangles Demo without Time Slicing

Animation and numbers updates timeline from the Triangles demo without time slicing.

As we can see from the picture above when a numbers update is triggered, Fiber will iterate through the whole tree looking for changes while building up the work-in-progress tree. This keeps the main thread busy for around 350ms.

Preventing the main thread from managing the animation updates causes a really long frame and, consequentially, the effects the user will see is a janky motion.

This is happening because we are using React Fiber on compatibility mode with React Stack. All the updates are not prioritised and they are treated synchronously.

The triangles demo with time slicing (smooth animation)

Now let’s analyse how Fiber can defer updates to avoid dropping frames. This functionality of Fiber is also known as Time slicing or async setState.

Triangles Demo with Time Slicing

Animation and numbers updates timeline from the Triangles demo with time slicing.

This time we are using the new unstable_deferredUpdates API to update numbers:

ReactDOM.unstable_deferredUpdates(() => {
  this.setState(state => ({ seconds: (state.seconds % 10) + 1 }));

One thing to keep in mind is that we are passing an updater function to setState rather than a plain object. This is necessary in order to defer updates. More about functional setState can be found in this article.

When a numbers update is triggered, as you can see from the picture, React just calls the requestIdleCallBack function and it does not even start to look for changes in the new tree.

Numbers update is now considered low priority and it is therefore deferred.

I guess you are wondering what is this requestIdleCallback function.


requestIdleCallback is a window method that provides a way to cooperate with the browser’s overall work schedule.

The callback argument we pass to requestIdleCallback is evaluated by the main thread as soon as it identifies an idle period. An idle period may be a few milliseconds between painting individual frames.

So requestIdleCallback gives Fiber an efficient way to understand when the main thread has some “spare time” to do some work.

Focusing again on the triangle demo, we can see on the picture a chunk of the numbers update reconciliation phase. The chunk of work starts when the idle callback event is fired: the browser is telling Fiber that it has some “spare time” to do work.

The browser also gives to Fiber a time remaining value which provides an estimate of the number of milliseconds remaining in the current idle period. When this time is over, Fiber knows it has to stop doing work and relinquish control back to the main thread which has something more important to do; in our case, an animation update.

Before passing control to the main thread, Fiber makes sure to call requestIdleCallback again because it just paused the reconciliation phase and it still has remaining work. It needs the main thread to come back to it as soon as it identifies a new idle period.

Numbers Update Chunk

Numbers update chunk timeline from the Triangles demo with time slicing.

The image above shows a chunk of the numbers update reconciliation phase. Fiber managed to update 18 dots out of 729 in this particular chunk of work. This suggests the work has been split into around 40 chunks of work, allowing the main thread to handle the animation update and repainting 40 times.

This way of “collaborating” with the main thread in order to do work is known under the name of Cooperative Scheduling and it takes inspiration from the operative systems world.

So thanks to cooperative scheduling, Fiber is able to interrupt work when the main thread needs to take care of something more urgent, and this, for the triangles demo, reflects a smoother animation without dropping frames.

When will React Fiber be released ?

Really soon in the next major version and without changing the public API.

Facebook folks care a lot about stability. They do not like breaking changes because they have something like 30.000 components to maintain.

Fiber will be enabled by default in React 16.

The first releases will focus on backward compatibility and not on new features.

Facebook is already using React Fiber in production. You can navigate to Facebook, open the Dev Tools and type require('React').version to find out the exact version of React they are using.

You can start play with React Fiber beta today by simply typing:

What new features will React Fiber introduce ?

React Fiber opens the door to a lot of new functionalities. These are the ones the React Core team has already started working on.

NB: Some of these new features will not be available in React 16 which, as I already said, will mainly focus on backward compatibility.


So in a nutshell, why should you be excited about this shiny new version of the React reconciler algorithm?

  • It makes apps more fluid and responsible allowing high priority updates to jump ahead of low priority updates. It does that by breaking up the work into small units of work that can be paused.
  • In the future it could parallelise work between multiple workers in an efficient way by splitting up branches of the tree and analysing them in parallel.
  • In the long term it would improve startup time rendering components as they become available to the browser without the need to wait for a whole bundle to be downloaded.

The future looks bright and full of new possibilities for React


Giamir Buoncristiani

Giamir Buoncristiani

Software Developer @ThoughtWorks.
Agile methodologies devotee. JavaScript lover.