An expert would know the ecma spec inside and out, know what javascript does differently from similar procedural languages, know how to approach performance and

An expert would know the ecma spec inside and out, know what javascript does differently from similar procedural languages, know how to approach performance and memory management issues, know common practical development patterns, know the DOM and the new HTML5 APIs, know the differences in the implemented capabilities of commonly used browsers, and have studied (or helped write) one of the open-source implementations.