Worked on the vuejs/core repository to address a targeted issue in the reactivity system, focusing on preserving the return value of a watch callback when the once option is enabled. The solution involved modifying the internal wrapper function to correctly capture and return the callback’s result, ensuring compatibility with asynchronous workflows and reducing unexpected behavior for downstream components. Comprehensive unit tests were added to verify the fix and improve async error handling. This work, implemented using TypeScript and leveraging expertise in reactivity systems and unit testing, enhanced the reliability of reactive features without altering the public API surface or introducing regressions.