We’re excited to announce the release of React v16.0! Among the changes are some long-standing feature requests, including fragments, error boundaries, portals, support for custom DOM attributes, improved server-side rendering, and reduced file size.
Previously, runtime errors during rendering could put React in a broken state, producing cryptic error messages and requiring a page refresh to recover. To address this problem, React 16 uses a more resilient error-handling strategy. By default, if an error is thrown inside a component’s render or lifecycle methods, the whole component tree is unmounted from the root. This prevents the display of corrupted data. However, it’s probably not the ideal user experience.
Instead of unmounting the whole app every time there’s an error, you can use error boundaries. Error boundaries are special components that capture errors inside their subtree and display a fallback UI in its place. Think of error boundaries like try-catch statements, but for React components.
React 16 includes a completely rewritten server renderer. It’s really fast. It supports streaming, so you can start sending bytes to the client faster. And thanks to a new packaging strategy that compiles away process.env checks (Believe it or not, reading process.env in Node is really slow!), you no longer need to bundle React to get good server-rendering performance.
Core team member Sasha Aickin wrote a great article describing React 16’s SSR improvements. According to Sasha’s synthetic benchmarks, server rendering in React 16 is roughly three times faster than React 15. “When comparing against React 15 with process.env compiled out, there’s about a 2.4x improvement in Node 4, about a 3x performance improvement in Node 6, and a full 3.8x improvement in the new Node 8.4 release. And if you compare against React 15 without compilation, React 16 has a full order of magnitude gain in SSR in the latest version of Node!” (As Sasha points out, please be aware that these numbers are based on synthetic benchmarks and may not reflect real-world performance.)
In addition, React 16 is better at hydrating server-rendered HTML once it reaches the client. It no longer requires the initial render to exactly match the result from the server. Instead, it will attempt to reuse as much of the existing DOM as possible. No more checksums! In general, we don’t recommend that you render different content on the client versus the server, but it can be useful in some cases (e.g. timestamps).
Instead of ignoring unrecognized HTML and SVG attributes, React will now pass them through to the DOM. This has the added benefit of allowing us to get rid of most of React’s attribute whitelist, resulting in reduced file sizes.
・react is 5.3 kb (2.2 kb gzipped), down from 20.7 kb (6.9 kb gzipped).
・react-dom is 103.7 kb (32.6 kb gzipped), down from 141 kb (42.9 kb gzipped).
・react + react-dom is 109 kb (34.8 kb gzipped), down from 161.7 kb (49.8 kb gzipped).
The size difference is partly attributable to a change in packaging. React now uses Rollup to create flat bundles for each of its different target formats, resulting in both size and runtime performance wins. The flat bundle format also means that React’s impact on bundle size is roughly consistent regardless of how you ship your app, whether it’s with Webpack, Browserify, the pre-built UMD bundles, or any other system.
React 16 is the first version of React built on top of a new core architecture, codenamed “Fiber.” You can read all about this project over on Facebook’s engineering blog. (Spoiler: we rewrote React!)
React 16は「Fiber」というコードネームがつけられていた新しいコアアーキテクチャの上に構築された初めてのバージョンです。Facebookのエンジニアブログを通してこのプロジェクトについての全てを読むことができます。（スポイラー: we rewrote React!）。
Fiber is responsible for most of the new features in React 16, like error boundaries and fragments. Over the next few releases, you can expect more new features as we begin to unlock the full potential of React.
Perhaps the most exciting area we’re working on is async rendering—a strategy for cooperatively scheduling rendering work by periodically yielding execution to the browser. The upshot is that, with async rendering, apps are more responsive because React avoids blocking the main thread.
This demo provides an early peek at the types of problems async rendering can solve:
We think async rendering is a big deal, and represents the future of React. To make migration to v16.0 as smooth as possible, we’re not enabling any async features yet, but we’re excited to start rolling them out in the coming months. Stay tuned!
Although React 16 includes significant internal changes, in terms of upgrading, you can think of this like any other major React release. We’ve been serving React 16 to Facebook and Messenger.com users since earlier this year, and we released several beta and release candidate versions to flush out additional issues. With minor exceptions, if your app runs in 15.6 without any warnings, it should work in 16.
Hydrating a server-rendered container now has an explicit API. If you’re reviving server-rendered HTML, use ReactDOM.hydrate instead of ReactDOM.render. Keep using ReactDOM.render if you’re just doing client-side rendering.
As previously announced, we’ve discontinued support for React Addons. We expect the latest version of each addon (except react-addons-perf; see below) to work for the foreseeable future, but we won’t publish additional updates.
react-addons-perf no longer works at all in React 16. It’s likely that we’ll release a new version of this tool in the future. In the meantime, you can use your browser’s performance tools to profile React components.
React 15 had limited, undocumented support for error boundaries using unstable_handleError. This method has been renamed to componentDidCatch. You can use a codemod to automatically migrate to the new API.
It is not safe to re-render into a container that was modified by something other than React. This worked previously in some cases but was never supported. We now emit a warning in this case. Instead you should clean up your component trees using ReactDOM.unmountComponentAtNode. See this example.
There is no react/lib/* and react-dom/lib/* anymore. Even in CommonJS environments, React and ReactDOM are precompiled to single files (“flat bundles”). If you previously relied on undocumented React internals, and they don’t work anymore, let us know about your specific case in a new issue, and we’ll try to figure out a migration strategy for you.
The deprecations introduced in 15.x have been removed from the core package. React.createClass is now available as create-react-class, React.PropTypes as prop-types, React.DOM as react-dom-factories, react-addons-test-utils as react-dom/test-utils, and shallow renderer as react-test-renderer/shallow. See 15.5.0 and 15.6.0 blog posts for instructions on migrating code and automated codemods.
The names and paths to the single-file browser builds have changed to emphasize the difference between development and production builds. For example:
・react/dist/react.js → react/umd/react.development.js
・react/dist/react.min.js → react/umd/react.production.min.js
・react-dom/dist/react-dom.js → react-dom/umd/react-dom.development.js
・react-dom/dist/react-dom.min.js → react-dom/umd/react-dom.production.min.js
React 16 depends on the collection types Map and Set. If you support older browsers and devices which may not yet provide these natively (e.g. IE < 11), consider including a global polyfill in your bundled application, such as core-js or babel-polyfill.