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.json contains 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.js is an node script that traverses our codebase and updates codes.json. You can test it by running yarn 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. with yarn download build). It works with partial builds, too.
• transform-error-messages is a Babel pass that rewrites error messages to IDs for a production (minified) build.