Andrew Clark
7142d110b0
* Handle render phase updates explicitly We fire a warning in development if a component is updated during the render phase (with the exception of local hook updates, which have their own defined behavior). Because it's not a supported React pattern, we don't have that many tests that trigger this path. But it is meant to have reasonable semantics when it does happen, so that if it accidentally ships to production, the app doesn't crash unnecessarily. The behavior is not super well-defined, though. There are also some _internal_ React implementation details that intentionally to rely on this behavior. Most prominently, selective hydration and useOpaqueIdentifier. I need to tweak the behavior of render phase updates slightly as part of a fix for useOpaqueIdentifier. This shouldn't cause a user-facing change in behavior outside of useOpaqueIdentifier, but it does require that we explicitly model render phase updates. * Bugfix: Nested useOpaqueIdentifier calls Fixes an issue where multiple useOpaqueIdentifier hooks are upgraded to client ids within the same render. The way the upgrade works is that useOpaqueIdentifier schedules a render phase update then throws an error to trigger React's error recovery mechanism. The normal error recovery mechanism is designed for errors that occur as a result of interleaved mutations, so we usually only retry a single time, synchronously, before giving up. useOpaqueIdentifier is different because the error its throws when upgrading is not caused by an interleaved mutation. Rather, it happens when an ID is referenced for the first time inside a client-rendered tree (i.e. sommething that wasn't part of the initial server render). The fact that it relies on the error recovery mechanism is an implementation detail. And a single recovery attempt may be insufficient. For example, if a parent and a child component may reference different ids, and both are mounted as a result of the same client update, that will trigger two separate error recovery attempts. Because render phase updates are not allowed when triggered from userspace — we log a warning in developement to prevent them — we can assume that if something does update during the render phase, it is one of our "legit" implementation details like useOpaqueIdentifier. So we can keep retrying until we succeed — up to a limit, to protect against inifite loops. I chose 50 since that's the limit we use for commit phase updates.
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 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 which have a relatively limited scope. This is a great place to get started.
License
React is MIT licensed.