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* Pass children to hydration root constructor I already made this change for the concurrent root API in #23309. This does the same thing for the legacy API. Doesn't change any behavior, but I will use this in the next steps. * Add isRootDehydrated function Currently this does nothing except read a boolean field, but I'm about to change this logic. Since this is accessed by React DOM, too, I put the function in a separate module that can be deep imported. Previously, it was accessing the FiberRoot directly. The reason it's a separate module is to break a circular dependency between React DOM and the reconciler. * Allow updates at lower pri without forcing client render Currently, if a root is updated before the shell has finished hydrating (for example, due to a top-level navigation), we immediately revert to client rendering. This is rare because the root is expected is finish quickly, but not exceedingly rare because the root may be suspended. This adds support for updating the root without forcing a client render as long as the update has lower priority than the initial hydration, i.e. if the update is wrapped in startTransition. To implement this, I had to do some refactoring. The main idea here is to make it closer to how we implement hydration in Suspense boundaries: - I moved isDehydrated from the shared FiberRoot object to the HostRoot's state object. - In the begin phase, I check if the root has received an by comparing the new children to the initial children. If they are different, we revert to client rendering, and set isDehydrated to false using a derived state update (a la getDerivedStateFromProps). - There are a few places where we used to set root.isDehydrated to false as a way to force a client render. Instead, I set the ForceClientRender flag on the root work-in-progress fiber. - Whenever we fall back to client rendering, I log a recoverable error. The overall code structure is almost identical to the corresponding logic for Suspense components. The reason this works is because if the update has lower priority than the initial hydration, it won't be processed during the hydration render, so the children will be the same. We can go even further and allow updates at _higher_ priority (though not sync) by implementing selective hydration at the root, like we do for Suspense boundaries: interrupt the current render, attempt hydration at slightly higher priority than the update, then continue rendering the update. I haven't implemented this yet, but I've structured the code in anticipation of adding this later. * Wrap useMutableSource logic in feature flag
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 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:
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 that have a relatively limited scope. This is a great place to get started.
License
React is MIT licensed.