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This adds an experimental hook tentatively called useOptimisticState. (The actual name needs some bikeshedding.) The headline feature is that you can use it to implement optimistic updates. If you set some optimistic state during a transition/action, the state will be automatically reverted once the transition completes. Another feature is that the optimistic updates will be continually rebased on top of the latest state. It's easiest to explain with examples; we'll publish documentation as the API gets closer to stabilizing. See tests for now. Technically the use cases for this hook are broader than just optimistic updates; you could use it implement any sort of "pending" state, such as the ones exposed by useTransition and useFormStatus. But we expect people will most often reach for this hook to implement the optimistic update pattern; simpler cases are covered by those other hooks.
The error code system substitutes React's error messages with error IDs to provide a better debugging support in production. Check out the blog post here.
codes.jsoncontains the mapping from IDs to error messages. This file is generated by the Gulp plugin and is used by both the Babel plugin and the error decoder page in our documentation. This file is append-only, which means an existing code in the file will never be changed/removed.extract-errors.jsis an node script that traverses our codebase and updatescodes.json. You can test it by runningyarn extract-errors. It works by crawling the build artifacts directory, so you need to have either run the build script or downloaded pre-built artifacts (e.g. withyarn download build). It works with partial builds, too.transform-error-messagesis a Babel pass that rewrites error messages to IDs for a production (minified) build.