Andrew Clark
85bb7b685b
This fixes the "double free" bug illustrated by the regression test added in the previous commit. The underlying issue is that `effect.destroy` field is a mutable field but we read it during render. This is a concurrency bug — if we had a borrow checker, it would not allow this. It's rare in practice today because the field is updated during the commit phase, which takes a lock on the fiber tree until all the effects have fired. But it's still theoretically wrong because you can have multiple Fiber copies each with their own reference to a single destroy function, and indeed we discovered in production a scenario where this happens via our current APIs. In the future these types of scenarios will be much more common because we will introduce features where effects may run concurrently with the render phase — i.e. an imperative `hide` method that synchronously hides a React tree and unmounts all its effects without entering the render phase, and without interrupting a render phase that's already in progress. A future version of React may also be able to run the entire commit phase concurrently with a subsequent render phase. We can't do this now because our data structures are not fully thread safe (see: the Fiber alternate model) but we should be able to do this in the future. The fix I've introduced in this commit is to move the `destroy` field to a separate object. The effect "instance" is a shared object that remains the same for the entire lifetime of an effect. In Rust terms, a RefCell. The field is `undefined` if the effect is unmounted, or if the effect ran but is not stateful. We don't explicitly track whether the effect is mounted or unmounted because that can be inferred by the hiddenness of the fiber in the tree, i.e. whether there is a hidden Offscreen fiber above it. It's unfortunate that this is stored on a separate object, because it adds more memory per effect instance, but it's conceptually sound. I think there's likely a better data structure we could use for effects; perhaps just one array of effect instances per fiber. But I think this is OK for now despite the additional memory and we can follow up with performance optimizations later. --------- Co-authored-by: Dan Abramov <dan.abramov@gmail.com> Co-authored-by: Rick Hanlon <rickhanlonii@gmail.com> Co-authored-by: Jan Kassens <jan@kassens.net>
React · 
React is a JavaScript library for building user interfaces.
- Declarative: React makes it painless to create interactive UIs. Design simple views for each state in your application, and React will efficiently update and render just the right components when your data changes. Declarative views make your code more predictable, simpler to understand, and easier to debug.
- Component-Based: Build encapsulated components that manage their own state, then compose them to make complex UIs. Since component logic is written in JavaScript instead of templates, you can easily pass rich data through your app and keep the state out of the DOM.
- Learn Once, Write Anywhere: We don't make assumptions about the rest of your technology stack, so you can develop new features in React without rewriting existing code. React can also render on the server using Node and power mobile apps using React Native.
Learn how to use React in your project.
Installation
React has been designed for gradual adoption from the start, and you can use as little or as much React as you need:
- Use Online Playgrounds to get a taste of React.
- Add React to a Website as a
<script>tag in one minute. - Create a New React App if you're looking for a powerful JavaScript toolchain.
You can use React as a <script> tag from a CDN, or as a react package on npm.
Documentation
You can find the React documentation on the website.
Check out the Getting Started page for a quick overview.
The documentation is divided into several sections:
You can improve it by sending pull requests to this repository.
Examples
We have several examples on the website. Here is the first one to get you started:
import { createRoot } from 'react-dom/client';
function HelloMessage({ name }) {
return <div>Hello {name}</div>;
}
const root = createRoot(document.getElementById('container'));
root.render(<HelloMessage name="Taylor" />);
This example will render "Hello Taylor" into a container on the page.
You'll notice that we used an HTML-like syntax; we call it JSX. JSX is not required to use React, but it makes code more readable, and writing it feels like writing HTML. If you're using React as a <script> tag, read this section on integrating JSX; otherwise, the recommended JavaScript toolchains handle it automatically.
Contributing
The main purpose of this repository is to continue evolving React core, making it faster and easier to use. Development of React happens in the open on GitHub, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving React.
Code of Conduct
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.
Contributing Guide
Read our contributing guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React.
Good First Issues
To help you get your feet wet and get you familiar with our contribution process, we have a list of good first issues that contain bugs that have a relatively limited scope. This is a great place to get started.
License
React is MIT licensed.