Worked on the tc39/test262 repository to enhance the robustness and correctness of JavaScript core builtins by focusing on test coverage improvements. Addressed a specific bug related to error propagation in String.fromCharCode when handling BigInt inputs, ensuring that abrupt completions from the ToNumber conversion are properly surfaced at the call site. Used JavaScript and testing methodologies to validate that error paths involving ToNumber and ToUint16 conversions align with ECMAScript semantics. This targeted work improved the reliability and maintainability of test262, supporting future bug fixes and ensuring the suite accurately reflects observable error behavior in the language specification.