Are javascript viruses dangerous?
Can JavaScript malware be as dangerous and damaging as more traditional viruses that are programmed mostly in native APIs?
The threat a virus imposes on your system is, ideally, independent of its programming language because viruses exploit vulnerabilities in operating systems, applications, APIs etc. In this sense, a Javascript virus is as dangerous as any other virus. Also, for web applications, JavaScript is one of the main attack vectors in techniques such as cross-site scripting (XSS) and cross-site request forgery (CSRF) because it's rather easy to write for the web. Cisco's Annual Security Report and OWASP Top 10 rank XSS one of the most used methods for vulnerability exploitation.
tl;dr: JavaScript viruses are as dangerous as any other programming language.