Andrew Clark
8fe066fdac
* Bugfix: "Captured" updates on legacy queue This fixes a bug with error boundaries. Error boundaries have a notion of "captured" updates that represent errors that are thrown in its subtree during the render phase. These updates are meant to be dropped if the render is aborted. The bug happens when there's a concurrent update (an update from an interleaved event) in between when the error is thrown and when the error boundary does its second pass. The concurrent update is transferred from the pending queue onto the base queue. Usually, at this point the base queue is the same as the current queue. So when we append the pending updates to the work-in-progress queue, it also appends to the current queue. However, in the case of an error boundary's second pass, the base queue has already forked from the current queue; it includes both the "captured" updates and any concurrent updates. In that case, what we need to do is append separately to both queues. Which we weren't doing. That isn't the full story, though. You would expect that this mistake would manifest as dropping the interleaved updates. But instead what was happening is that the "captured" updates, the ones that are meant to be dropped if the render is aborted, were being added to the current queue. The reason is that the `baseQueue` structure is a circular linked list. The motivation for this was to save memory; instead of separate `first` and `last` pointers, you only need to point to `last`. But this approach does not work with structural sharing. So what was happening is that the captured updates were accidentally being added to the current queue because of the circular link. To fix this, I changed the `baseQueue` from a circular linked list to a singly-linked list so that we can take advantage of structural sharing. The "pending" queue, however, remains a circular list because it doesn't need to be persistent. This bug also affects the root fiber, which uses the same update queue implementation and also acts like an error boundary. It does not affect the hook update queue because they do not have any notion of "captured" updates. So I've left it alone for now. However, when we implement resuming, we will have to account for the same issue. * Ensure base queue is a clone When an error boundary captures an error, we append the error update to the work-in-progress queue only so that if the render is aborted, the error update is dropped. Before appending to the queue, we need to make sure the queue is a work-in-progress copy. Usually we clone the queue during `processUpdateQueue`; however, if the base queue has lower priority than the current render, we may have bailed out on the boundary fiber without ever entering `processUpdateQueue`. So we need to lazily clone the queue. * Add warning to protect against refactor hazard The hook queue does not have resuming or "captured" updates, but if we ever add them in the future, we'll need to make sure we check if the queue is forked before transfering the pending updates to them.
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 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 own 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:
function HelloMessage({ name }) {
return <div>Hello {name}</div>;
}
ReactDOM.render(
<HelloMessage name="Taylor" />,
document.getElementById('container')
);
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 to evolve 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 which have a relatively limited scope. This is a great place to get started.
License
React is MIT licensed.