A Beginner's Guide to React Fiber

A Beginner's Guide to React Fiber

React Fiber is a set of internal algorithms for rendering graphics used by the JavaScript library React, as opposed to its old rendering algorithm, Stack.

Before we understand how & what fiber is, let us understand what exactly happens when you want to render something on the Browser using react

It includes 2 main components

  • Reconciler
  • Renderer

All the changes that needs to be applied to the current tree of React Elements to reflect the updated state on the browser is determined by the reconciler during reconciliation

Note : The browser doesn’t reflect the changes made during reconciliation


Once your tree is ready you need to apply these changes on the browser, renderer does that for you. Renderer updates your rendered app by taking care of all the platform specific calls.

Example: react-dom which updates the DOM. react-native is another popular renderer. These are just few of the many pluggable renderers available.

P.S – You can also create your own custom renderer, isn’t that great!!

When it comes to Fiber, its actually a re-implementaion of the React’s core reconciliation algorithm.

But why to re-write something from scratch?

In order to understand this we need to know how React’s old reconciliation algorithm worked. Let me explain you in a nutshell.

You only have one main thread in javascript that does your UI updation, state change, network computation & responding to user’s action.

  • At any point in time React has 2 tree’s (VDOM tree) in memory, the current tree which is already rendered and the updated tree with all the latest changes.
  • The Stack reconciler finds the difference between 2 tree’s synchronously in a single pass
  • This prevents the main thread from doing potential urgent work until the recursive process has finished.
  • If user happens to type in a text input during this phase, your app becomes unresponsive as main thread is busy. A similar kind of thing happens when an animation needs to happen and your main thread is busy, that’s when you experience Jank.

This had to be solved.

This is where fiber comes into picture

So What is Fiber?

Fiber is a new data structure which represents a unit of work

In simple terms it is a javascript object that maintains a one to one relationship with the react instances

Rendering the Fiber Tree

The first fiber node which react creates is the host root which represents the container dom node ( the dom element which you pass to ReactDOM.render() )

Fiber object has specific properties which allows it to keep track of information and relationship between fiber nodes



stateNode keeps reference to the component instance fiber belongs to

child, sibling & return represents the child, siblings and the parent node with respect to the current fiber node

type determines if its a class or function or DOM element

alternate holds the reference between the nodes in current tree and work-in-progress tree

key identifies all the changed, added, or removed elements

updateQueue queues all the state & DOM updates or any other effect

memoizedState holds the reference to the state of the previous render

memoizedProps holds the reference to the props supplied to the previous render

pendingProps represents the new props passed for the current update

tag denotes the type of Fiber example: Class component, Function component, Host portal.

effectTag holds the information about the side-effect which needs to be applied

nextEffect points to the next node in the effects list which has an update

React starts creating a fiber tree upon initial render known as current fiber tree.

Whenever there is an update, React starts building the work in progress(WIP) fiber tree

Why WIP?

So that there are no partial changes in the DOM. Once all the work is calculated by React then all the DOM updates are applied together which is what makes React Fiber so performant and smooth.

In stack Reconciler as soon as the instance was updated, the DOM used to be updated without having any information about the subsequent changes which was making the UI inconsistent.

How does Fiber avoid UI inconsistency?

By simply splitting work in Phases

  1. Render / Reconciliation phase
  2. Commit phase

Phase 1 can be paused and resumed whereas Phase 2 must be completed in one go

In phase 1 react starts building the WIP tree, which goes about something like this

  • setState() method is called to update a component’s state
  • React knows it has to now schedule the work using requestIdleCallback() which lets the main thread know that it has to pick up the work once it has some free(Idle) time
  • React now starts creating the WIP Fiber tree by cloning the elements from the current Fiber tree and goes through each node to determine if it has to be changed
  • If a particular node has an update, it is added to another list called as effects list which is a linear linked list of all the changes that needs to be made
  • Once entire WIP tree is traversed and all the updated nodes are tagged, phase 1 is completed

In Phase 2 (commit phase), all the updates for the nodes in the effect list are performed and reflected on the DOM.The main thread applies all these changes in a single go

But how does it solve my jank problem??

One of the most differentiating feature in React Fiber is Prioritization. Fiber reconciler can prioritize different tasks which needs to be done.

So if your application is doing some data fetching while the user is typing something in the textbox, updating the UI to reflect users text would be given higher priority as compared to fetching the data.

Takeaway tip

Always use the updater function while working with React Fiber

setState() with a function as argument instead of object is an updater function

this.setState((state, props) => {
  return {

Using updater function always guarantees that the state and props will always be up-to-date


Fiber in nutshell allows React to better utilize the main thread by pausing/ resuming/ prioritising/ cancelling updates to provide a seamless user experience.

JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at https://medium.com

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI

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