Andrew Clark
dc27b5aaae
* Concurrent Mode test for uMS render mutation Same test as the one added in #20665, but for Concurrent Mode. * Use double render to detect render phase mutation PR #20665 added a mechanism to detect when a `useMutableSource` source is mutated during the render phase. It relies on the fact that we double invoke components that error during development using `invokeGuardedCallback`. If the version in the double render doesn't match the first, that indicates there must have been a mutation during render. At first I thought it worked by detecting inside the *other* double render, the one we do for Strict Mode. It turns out that while it does warn then, the warning is suppressed, because we suppress all console methods that occur during the Strict Mode double render. So it's really the `invokeGuardedCallback` one that makes it work. Anyway, let's set that aside that issue for a second. I realized during review that errors that occur during the Strict Mode double render reveal a useful property: A pure component will never throw during the double render, because if it were pure, it would have also thrown during the first render... in which case it wouldn't have double rendered! This is true of all such errors, not just the one thrown by `useMutableSource`. Given this, we can simplify the `useMutableSource` mutation detection mechanism. Instead of tracking and comparing the source's version, we can instead check if we're inside a double render when the error is thrown. To get around the console suppression issue, I changed the warning to an error. It errors regardless, in both dev and prod, so it doesn't have semantic implications. However, because of the paradox I described above, we arguably _shouldn't_ throw an error in development, since we know that error won't happen in production, because prod doesn't double render. (It's still a tearing bug, but that doesn't mean the component will actually throw.) I considered that, but that requires a larger conversation about how to handle errors that we know are only possible in development. I think we should probably be suppressing *all* errors (with a warning) that occur during a double render.
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.