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The old (unstable) mechanism for suspending was to throw a promise. The purpose of throwing is to interrupt the component's execution, and also to signal to React that the interruption was caused by Suspense as opposed to some other error. A flaw is that throwing is meant to be an implementation detail — if code in userspace catches the promise, it can lead to unexpected behavior. With `use`, userspace code does not throw promises directly, but `use` itself still needs to throw something to interrupt the component and unwind the stack. The solution is to throw an internal error. In development, we can detect whether the error was caught by a userspace try/catch block and log a warning — though it's not foolproof, since a clever user could catch the object and rethrow it later. The error message includes advice to move `use` outside of the try/catch block. I did not yet implement the warning in Flight.
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.