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Sebastian "Sebbie" SilbermannandGitHub
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When a rejected `Promise` is instrumented in userspace while setting the rejection reason in the wrong field (e.g. `error` instead of `reason`), React will throw undefined (because `usable.reason` doesn't exist). This makes it incredibly hard to find the actual rejection reason. React is now throwing a generic error if we couldn't find the rejection reason. That will produce a callstack pointing into the problematic Promise from where you can hopefully extract the real rejection reason (alongside fixing the bad instrumentation). We're doing this in prod since this is unlikely to surface in dev. We're only doing runtime type-checking for the rejected case. That should be hit rarely and therefore hopefully have negligible runtime impact.
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.