The interactions imposed by JavaScript can present accessibility barriers. A serious JavaScript developer is already familiar with WCAG 2.0 and knows how to work within its recommendations or when to push back on violating business requirements.
Security. You need to have at least a basic understanding of security violations, security controls, and privacy. You don't need to be a CISSP, but you need to be able to supply recommendations and avoid obvious failures. If you cannot get this right in
