Inside Fiber: the in-depth overview you wanted a TLDR for

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I want to provide an in-depth overview of the important concepts behind reconciliation. We'll then explore how React uses the algorithm and go through a few magic words we hear a lot, like coroutines, continuations, fibers, generators, algebraic effects and see how they all relate to React.js.

This talk has been presented at React Summit 2022, check out the latest edition of this React Conference.

FAQ

The speaker is Mateusz, a senior front-end engineer at Medaglia and a mentor at Tech Labs Berlin.

The main topic is 'Inside FIBR,' focusing on the architecture of React Fiber and its implications for front-end development.

Some keywords and concepts include fibers, coroutines, continuations, algebraic effects, threads, generators, and React's internal types.

A fiber is a stack frame for a given React component, allowing full control over the scheduling of what should be done.

During reconciliation, React tags elements based on their type (e.g., function component, class component, suspense component) and converts each element into a fiber node that describes the required work.

Coroutines are generators that can consume values, often resolving async values. In React, coroutines were initially introduced to give developers full control over pausing and resuming tasks.

React's multitasking comparative model allows rendering to be interruptible and interleaved with other tasks, improving UI responsiveness and performance.

Effect handlers in React allow the framework to handle specific tasks and run code in response to those tasks, similar to try-catch but for handling effects.

React can suspend rendering by throwing a promise, which is then caught and handled by the framework, enabling features like React Suspense.

The scheduler in React Fiber determines what runs next, allowing for tasks to be tracked, scheduled, paused, and aborted as needed.

Matheus Albuquerque
Matheus Albuquerque
27 min
17 Jun, 2022

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Video Summary and Transcription
This Talk explores the internals of React Fiber and its implications. It covers topics such as fibres and units of work, inspecting elements and parent matching, pattern matching and coroutines, and the influence of coroutines on concurrent React. The Talk also discusses effect handlers in React, handling side effects in components, and the history of effect handlers in React. It concludes by emphasizing the importance of understanding React internals and provides learning resources for further exploration.

1. Introduction to Inside FIBR

Short description:

Hi, everyone. This session is Inside FIBR, the TRDR you wanted for. I'm Mateusz, a senior front-end engineer at Medaglia and a mentor at Tech Labs Berlin. React source code is complex, but we can discuss it further. I wanted to present this because of my friend Bruno's proposal to bring algebraic effects into JavaScript. I'll explain the topics and their connection to React.

Hi, everyone. It's great to be here. Hello, Amsterdam. Hello, everyone who's online. Yeah, I'm Mateusz and this session is Inside FIBR, the TRDR you wanted for. This is me. You can find me everywhere as Ytacombinator and I'm a senior front-end engineer at Medaglia and I also mentor front-end people at Tech Labs Berlin.

By the way, we are hiding. Before we start, I have a few disclaimers. The first one is that, as you probably can imagine, React source code is really complex and some of the thoughts I have here are a bit speculative. And the second thing is that maybe it's going to be not 100% what you call a TRDR, but whenever you find the huge payload of content, you'll see this icon, which means we can discuss that in the afterparty or in the Q&A sessions and etc. So don't worry.

Before I start, I'd like to provide you a bit of context of why I wanted to present this and why I wanted to talk about these stuff. It all started with this friend of mine called Bruno. He was preparing this proposal to bring algebraic effects into JavaScript, so it was just like regular TC39 proposal, and he asked me for some feedback. When I was reading his proposal, I thought I saw a huge amount of topics I had never heard about any of them, and there was this huge cloud in my mind, like algebraic effects, coroutines, continuations, fibres, threads, generators, and etc, and what I wanted to do here was to put those in a way that it makes sense to go through them and also to show how they all pieced together and react.

2. Overview of Fibres and Units of Work

Short description:

Fibres in React are similar to stack frames in regular JavaScript functions. They represent units of work and allow for tracking, scheduling, pausing, and aborting specific types of work. During reconciliation, elements are merged into alpha fibre nodes, which describe the work to be done. Visualizing these units can be done through inspecting elements, providing valuable metadata about components.

So starting with fibres and a bit of an overview. The way I like to see fibres is let's think about a regular JavaScript function. So we have this add functions, it has two parameters, I'll just sum them. If you think of what a stack frame for that would look like, we would get something like this. We have a return, we have the function itself, we have a few parameters and the local variables being the numbers or results.

If we think about a React component, we can get to something similar. So we can think of a fibre in this way where we have our components instead of a function. Our props instead of our parameters and then our component state being our local variables. So to start, we can think of the fibre architecture as this React specific call stack model that basically gives full control over scheduling of what should be done. And, a fibre itself is basically a stack frame for a given React component. Okay, we got to this definition, we got to this parallel with regular stack frames, now we can start seeing fibres as units of work.

So let's think what happens with our components. So once our template goes through the JSX compiler, we end up with a bunch of elements that's what you note. What happens next is during reconciliation, the data from all of these elements is merged into three alpha fibre nodes, then we'll talk a bit more about them. And then, of course, depending on the type of, what is it, for example, it can be a function component, a class component, a suspense component, or whatever, so depending on the type of the thing, React itself needs to perform different types of work. So it's going to tag these. And then, each element is converted into this fibre node that we're talking about, that's going to describe for React what kind of work that needs to be done. So this leads us to this unit of work thing. And because fibres are units of work, it makes that convenient to track, schedule, pause, and abort specific types of work.

Now that we can abstract those in terms of units of work, we can also think about visualizing those units. And for this, I'd like to propose this first experiment that is inspecting elements. So let's take this simple app. I have just a few components with some local state and I have this button. I am incrementing and decrementing this local state and that's about it. If we start logging our fibres in the console, we are going to see this, a bunch of metadata about our components. And we'll see metadata about props, about state, about etc. And I know it was like really tiny but don't bother trying to read that. But I just wanted to see that there's a bunch of information there. And we can use, for example, this kind of information. So here's a piece of code where I'm using only five of these properties to iterate on those fibres.

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