Worked on the vitest-dev/vitest repository to enhance the robustness of its event handling system. Addressed a bug in the addEventHandler function by enabling it to accept null as the third parameter without causing runtime errors, thereby improving reliability for developers using this API. Applied TypeScript and front end development skills to implement the fix and introduced a regression test to ensure stability across edge cases. Focused on test-driven development and careful debugging, the work reduced the risk of exceptions in user code and contributed to a more stable core event system, ultimately supporting a better developer experience and product quality.