115cd12d9b
In CI, we run our test suite against multiple build configurations. For example, we run our tests in both dev and prod, and in both the experimental and stable release channels. This is to prevent accidental deviations in behavior between the different builds. If there's an intentional deviation in behavior, the test author must account for them. However, we currently don't run tests against the www builds. That's a problem, because it's common for features to land in www before they land anywhere else, including the experimental release channel. Typically we do this so we can gradually roll out the feature behind a flag before deciding to enable it. The way we test those features today is by mutating the `shared/ReactFeatureFlags` module. There are a few downsides to this approach, though. The flag is only overridden for the specific tests or test suites where you apply the override. But usually what you want is to run *all* tests with the flag enabled, to protect against unexpected regressions. Also, mutating the feature flags module only works when running the tests against source, not against the final build artifacts, because the ReactFeatureFlags module is inlined by the build script. Instead, we should run the test suite against the www configuration, just like we do for prod, experimental, and so on. I've added a new command, `yarn test-www`. It automatically runs in CI. Some of the www feature flags are dynamic; that is, they depend on a runtime condition (i.e. a GK). These flags are imported from an external module that lives in www. Those flags will be enabled for some clients and disabled for others, so we should run the tests against *both* modes. So I've added a new global `__VARIANT__`, and a new test command `yarn test-www-variant`. `__VARIANT__` is set to false by default; when running `test-www-variant`, it's set to true. If we were going for *really* comprehensive coverage, we would run the tests against every possible configuration of feature flags: 2 ^ numberOfFlags total combinations. That's not practical, though, so instead we only run against two combinations: once with `__VARIANT__` set to `true`, and once with it set to `false`. We generally assume that flags can be toggled independently, so in practice this should be enough. You can also refer to `__VARIANT__` in tests to detect which mode you're running in. Or, you can import `shared/ReactFeatureFlags` and read the specific flag you can about. However, we should stop mutating that module going forward. Treat it as read-only. In this commit, I have only setup the www tests to run against source. I'll leave running against build for a follow up. Many of our tests currently assume they run only in the default configuration, and break when certain flags are toggled. Rather than fix these all up front, I've hard-coded the relevant flags to the default values. We can incrementally migrate those tests later.
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.