To educate myself and the community, I spend a lot of time reverse-engineering web technologies and writing about my findings. In the last year, I’ve focused mostly on Angular sources which resulted in the biggest Angular publication on the web — Angular-In-Depth. Now the time has come to dive deep into React. Change detection has become the main area of my expertise in Angular, and with some patience and a lot of debugging, I hope to soon achieve that level in React.

In React, the mechanism of change detection is often referred to as reconciliation or rendering, and Fiber is its newest implementation. Due to the underlying architecture, it provides capabilities to implement many interesting features like performing non-blocking rendering, applying updates based on the priority and pre-rendering content in the background. These features are referred to as time-slicing in the Concurrent React philosophy.

Besides solving real problems of application developers, the internal implementation of these mechanisms has a wide appeal from the engineering perspective. There’s such a wealth of knowledge in the sources that will help us grow as developers.

If you Google “React Fiber” today you’re going to see quite a lot articles in the search results. All of them, though, except for the notes by Andrew Clark are pretty high-level explanations. In this article, I’ll refer to this resource and provide an elaborate explanation for some particularly important concepts in Fiber. Once we’ve finished, you’ll have enough knowledge to understand the work loop representation from a very good talk by Lin Clark at ReactConf 2017. That’s the one talk you need to see. But it’ll make a lot more sense to you after you’ve spent a little time in the sources.

This post opens a series on React’s Fiber internals. I’m about 70% through understanding the internal details of the implementation and have three more articles on reconciliation and rendering mechanism in the works.

Let’s get started!

Setting the background #

Fiber’s architecture has two major phases: reconciliation/render and commit. In the source code the reconciliation phase is mostly referred to as the “render phase”. This is the phase when React walks the tree of components and:

updates state and props,

calls lifecycle hooks,

retrieves the children from the component,

compares them to the previous children,

and figures out the DOM updates that need to be performed.

All these activities are referred to as work inside Fiber. The type of work that needs to be done depends on the type of the React Element. For example, for a Class Component React needs to instantiate a class, while it doesn't do it for a Functional Component . If interested, here you can see all types of work targets in Fiber. These activities are exactly what Andrew talks about here:

When dealing with UIs, the problem is that if too much work is executed all at once, it can cause animations to drop frames…

Now what about that ‘all at once’ part? Well, basically, if React is going to walk the entire tree of components synchronously and perform work for each component, it may run over 16 ms available for an application code to execute its logic. And this will cause frames to drop causing stuttering visual effects.

So this can be helped?

Newer browsers (and React Native) implement APIs that help address this exact problem…

The new API he talks about is the requestIdleCallback global function that can be used to queue a function to be called during a browser’s idle periods. Here’s how you would use it by itself:

requestIdleCallback((deadline)=>{ console.log(deadline.timeRemaining(), deadline.didTimeout) });

If I now open the console and execute the code above, Chrome logs 49.9 false . It basically tells me that I have 49.9 ms to do whatever work I need to do and I haven’t yet used up all allotted time, otherwise the deadline.didTimeout would be true . Keep in mind that timeRemaining can change as soon as a browser gets some work to do, so it should be constantly checked.

requestIdleCallback is actually a bit too restrictive and is not executed often enough to implement smooth UI rendering, so React team had to implement their own version.

Now if we put all the activities Reacts performs on a component into the function performWork , and use requestIdleCallback to schedule the work, our code could look like this:

requestIdleCallback((deadline) => { // while we have time, perform work for a part of the components tree while ((deadline.timeRemaining() > 0 || deadline.didTimeout) && nextComponent) { nextComponent = performWork(nextComponent); } });

We perform the work on one component and then return the reference to the next component to process. This would work, if not for one thing. You can’t process the entire tree of components synchronously, as in the previous implementation of the reconciliation algorithm. And that’s the problem Andrew talks about here:

in order to use those APIs, you need a way to break rendering work into incremental units

So to solve this problem, React had to re-implement the algorithm for walking the tree from the synchronous recursive model that relied on the built-in stack to an asynchronous model with linked list and pointers. And that’s what Andrew writes about here:

If you rely only on the [built-in] call stack, it will keep doing work until the stack is empty…Wouldn’t it be great if we could interrupt the call stack at will and manipulate stack frames manually? That’s the purpose of React Fiber.Fiber is re-implementation of the stack, specialized for React components. You can think of a single fiber as a virtual stack frame.

And that’s what I’m going to explain now.

A word about the stack #

I assume you’re all familiar with the notion of a call stack. This is what you see in your browser’s debugging tools if you pause code at a breakpoint. Here are a few relevant quotes and diagrams from Wikipedia:

In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program… the main reason for having call stack is to keep track of the point to which each active subroutine should return control when it finishes executing… A call stack is composed of stack frames… Each stack frame corresponds to a call to a subroutine which has not yet terminated with a return. For example, if a subroutine named DrawLine is currently running, having been called by a subroutine DrawSquare , the top part of the call stack might be laid out like in the adjacent picture.

Why is the stack relevant to React? #

As we defined in the first part of the article, Reacts walks the components tree during the reconciliation/render phase and performs some work for components. The previous implementation of the reconciler used the synchronous recursive model that relied on the built-in stack to walk the tree. The official doc on reconciliation describe this process and talk a lot about recursion:

By default, when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference.

If you think about it, each recursive call adds a frame to the stack. And it does so synchronously. Suppose we have the following tree of components:

Represented as objects with the render function. You can think of them as instances of components:

const a1 = {name: 'a1'}; const b1 = {name: 'b1'}; const b2 = {name: 'b2'}; const b3 = {name: 'b3'}; const c1 = {name: 'c1'}; const c2 = {name: 'c2'}; const d1 = {name: 'd1'}; const d2 = {name: 'd2'}; a1.render = () => [b1, b2, b3]; b1.render = () => []; b2.render = () => [c1]; b3.render = () => [c2]; c1.render = () => [d1, d2]; c2.render = () => []; d1.render = () => []; d2.render = () => [];

React needs to iterate the tree and perform work for each component. To simplify, the work to do is to log the name of the current component and retrieve its children. Here’s how we do it with recursion.

Recursive traversal #

The main function that iterates over the tree is called walk in the implementation below:

walk(a1); function walk(instance) { doWork(instance); const children = instance.render(); children.forEach(walk); } function doWork(o) { console.log(o.name); }

Here’s the output we’re getting:

a1, b1, b2, c1, d1, d2, b3, c2

If you don’t feel confident with recursions, check out my in-depth article on recursion.

A recursive approach is intuitive and well-suited for walking the trees. But as we discovered, it has limitations. The biggest one is that we can’t break the work into incremental units. We can’t pause the work at a particular component and resume it later. With this approach React just keeps iterating until it processed all components and the stack is empty.

So how does React implement the algorithm to walk the tree without recursion? It uses a singly linked list tree traversal algorithm. It makes it possible to pause the traversal and stop the stack from growing.

Linked list traversal #

I was lucky to find the gist of the algorithm outlined by Sebastian Markbåge here. To implement the algorithm, we need to have a data structure with 3 fields:

child — reference to the first child

sibling — reference to the first sibling

return — reference to the parent

In the context of the new reconciliation algorithm in React, the data structure with these fields is called Fiber. Under the hood it’s the representation of a React Element that keeps a queue of work to do. More on that in my next articles.

The following diagram demonstrates the hierarchy of objects linked through the linked list and the types of connections between them:

So let’s first define our custom node constructor:

class Node { constructor(instance) { this.instance = instance; this.child = null; this.sibling = null; this.return = null; } }

And the function that takes an array of nodes and links them together. We’re going to use it to link children returned by the render method:

function link(parent, elements) { if (elements === null) elements = []; parent.child = elements.reduceRight((previous, current) => { const node = new Node(current); node.return = parent; node.sibling = previous; return node; }, null); return parent.child; }

The function iterates over the array of nodes starting from the last one and links them together in a singly linked list. It returns the reference to the first sibling in the list. Here is a simple demo of how it works:

const children = [{name: 'b1'}, {name: 'b2'}]; const parent = new Node({name: 'a1'}); const child = link(parent, children); // the following two statements are true console.log(child.instance.name === 'b1'); console.log(child.sibling.instance === children[1]);

We’ll also implement a helper function that performs some work for a node. In our case, it’s going to log the name of a component. But besides that it also retrieves the children of a component and links them together:

function doWork(node) { console.log(node.instance.name); const children = node.instance.render(); return link(node, children); }

Okay, now we’re ready to implement the main traversal algorithm. It’s a parent first, depth-first implementation. Here is the code with useful comments:

function walk(o) { let root = o; let current = o; while (true) { // perform work for a node, retrieve & link the children let child = doWork(current); // if there's a child, set it as the current active node if (child) { current = child; continue; } // if we've returned to the top, exit the function if (current === root) { return; } // keep going up until we find the sibling while (!current.sibling) { // if we've returned to the top, exit the function if (!current.return || current.return === root) { return; } // set the parent as the current active node current = current.return; } // if found, set the sibling as the current active node current = current.sibling; } }

Although the implementation is not particularly difficult to understand, you may need to play with it a little to grok it. Do it here. The idea is that we keep the reference to the current node and re-assign it while descending the tree until we hit the end of the branch. Then we use the return pointer to return to the common parent.

If we now check the call stack with this implementation, here’s what we’re going to see:

As you can see, the stack doesn’t grow as we walk down the tree. But if now put the debugger into the doWork function and log node names, we’re going to see the following:

It looks like a callstack in a browser. So with this algorithm, we’re effectively replacing the browser’s implementation of the call stack with our own implementation. That’s what Andrew describes here:

Fiber is re-implementation of the stack, specialized for React components. You can think of a single fiber as a virtual stack frame.

Since we’re now controlling the stack by keeping the reference to the node that acts as a top frame:

function walk(o) { let root = o; let current = o; while (true) { ... current = child; ... current = current.return; ... current = current.sibling; } }

we can stop the traversal at any time and resume to it later. That’s exactly the condition we wanted to achieve to be able to use the new requestIdleCallback API.

Work loop in React #

Here’s the code that implements work loop in React:

function workLoop(isYieldy) { if (!isYieldy) { // Flush work without yielding while (nextUnitOfWork !== null) { nextUnitOfWork = performUnitOfWork(nextUnitOfWork); } } else { // Flush asynchronous work until the deadline runs out of time. while (nextUnitOfWork !== null && !shouldYield()) { nextUnitOfWork = performUnitOfWork(nextUnitOfWork); } } }

As you can see, it maps nicely to the algorithm I presented above. It keeps the reference to the current fiber node in the nextUnitOfWork variable that acts as a top frame.

The algorithm can walk the components tree synchronously and perform the work for each fiber node in the tree (nextUnitOfWork). This is usually the case for so-called interactive updates caused by UI events (click, input etc). Or it can walk the components tree asynchronously checking if there’s time left after performing work for a Fiber node. The function shouldYield returns the result based on deadlineDidExpire and deadline variables that are constantly updated as React performs work for a fiber node.