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Adds a third argument called `getServerSnapshot`. On the server, React calls this one instead of the normal `getSnapshot`. We also call it during hydration. So it represents the snapshot that is used to generate the initial, server-rendered HTML. The purpose is to avoid server-client mismatches. What we render during hydration needs to match up exactly with what we render on the server. The pattern is for the server to send down a serialized copy of the store that was used to generate the initial HTML. On the client, React will call either `getSnapshot` or `getServerSnapshot` on the client as appropriate, depending on whether it's currently hydrating. The argument is optional for fully client rendered use cases. If the user does attempt to omit `getServerSnapshot`, and the hook is called on the server, React will abort that subtree on the server and revert to client rendering, up to the nearest Suspense boundary. For the userspace shim, we will need to use a heuristic (canUseDOM) to determine whether we are in a server environment. I'll do that in a follow up.
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.transform-error-messagesis a Babel pass that rewrites error messages to IDs for a production (minified) build.