Andrew Clark
d1547defe3
(Unless "key" is spread onto the element.) Historically, the JSX runtime clones the props object that is passed in. We've done this for two reasons. One reason is that there are certain prop names that are reserved by React, like `key` and (before React 19) `ref`. These are not actual props and are not observable by the target component; React uses them internally but removes them from the props object before passing them to userspace. The second reason is that the classic JSX runtime, `createElement`, is both a compiler target _and_ a public API that can be called manually. Therefore, we can't assume that the props object that is passed into `createElement` won't be mutated by userspace code after it is passed in. However, the new JSX runtime, `jsx`, is not a public API — it's solely a compiler target, and the compiler _will_ always pass a fresh, inline object. So the only reason to clone the props is if a reserved prop name is used. In React 19, `ref` is no longer a reserved prop name, and `key` will only appear in the props object if it is spread onto the element. (Because if `key` is statically defined, the compiler will pass it as a separate argument to the `jsx` function.) So the only remaining reason to clone the props object is if `key` is spread onto the element, which is a rare case, and also triggers a warning in development. In a future release, we will not remove a spread key from the props object. (But we'll still warn.) We'll always pass the object straight through. The expected impact is much faster JSX element creation, which in many apps is a significant slice of the overall runtime cost of rendering.
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 Quick Start to get a taste of React.
- Add React to an Existing Project to use as little or as much React as you need.
- Create a New React App if you're looking for a powerful JavaScript toolchain.
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:
- Quick Start
- Tutorial
- Thinking in React
- Installation
- Describing the UI
- Adding Interactivity
- Managing State
- Advanced Guides
- API Reference
- Where to Get Support
- Contributing Guide
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.
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.