This release includes a new class component lifecycle (`getDerivedStateFromProps`), a new `StrictMode` component, an official context API, and a new ergonomic ref API!
For the past few months, the React team has been working on support for [asynchronous rendering](/blog/2018/03/01/sneak-peek-beyond-react-16.html). We are excited about the new features it will enable.
We've also learned that some long-term changes will be required to the way we write React components. However, we respect semver and **will not ship breaking changes in a minor version**!
For many years, React has offered an experimental API for context. Although it was a powerful tool, its use was discouraged because of inherent problems in the API, and because we always intended to replace the experimental API with a better one.
Previously, React provided two ways for managing refs: the legacy string ref API and the callback API. Although the string ref API was the more convenient of the two, it had [several downsides](https://github.com/facebook/react/issues/1373) and so our official recomendation was to use the callback form instead.
React's class component API has been around for years with little change. However, as we add support for more advanced features (such as [error boundaries](/docs/react-component.html#componentdidcatch) and the upcoming [async rendering mode](/blog/2018/03/01/sneak-peek-beyond-react-16.html)) we stretch this model in ways that it was not originally intended.
For example, with the current API, it is too easy to block the initial render with non-essential logic. In part this is because there are too many ways to accomplish a given task, and it can be unclear which is best. We've observed that the interrupting behavior of error handling is often not taken into consideration and can result in memory leaks (something that will also impact the upcoming async rendering mode). The current class component API also complicates other efforts, like our work on [prototyping a React compiler](https://twitter.com/trueadm/status/944908776896978946).
Many of these issues are exacerbated by a subset of the component lifecycles (`componentWillMount`, `componentWillReceiveProps`, and `componentWillUpdate`). These also happen to be the lifecycles that cause the most confusion within the React community. For these reasons, we are going to deprecate those methods in favor of better alternatives.
We recognize that this change will impact many existing components. (At Facebook, we maintain more than 50,000 React components, and we can't tell our engineers to rewrite them either.) Because of this, the migration path will be as gradual as possible, and will provide escape hatches.
> Even in version 17, it will still be possible to use them, but they will be aliased with an "UNSAFE_" prefix to indicate that they might cause issues. We have also prepared an [automated script to rename them](https://github.com/reactjs/react-codemod#rename-unsafe-lifecycles) in existing code.
`StrictMode` is a tool for highlighting potential problems in an application. Like `Fragment`, `StrictMode` does not render any visible UI. It activates additional checks and warnings for its descendants.
Although it is not possible for strict mode to catch all problems (e.g. certain types of mutation), it can help with many. If you see warnings in strict mode, those things will likely cause bugs for async rendering.
