You can publish as much of your own javascript code as you want under any license on your website or github, or anywhere. The issue they have is not with the ownership of javascript the language. The issue is that if you wanted to take the frontend code to Facebook, Google Maps, Office 365, or something like that, and modify it to create your own version, you have two problems. One, that it probably isn't allowed by the license and the Facebook/Google/Microsoft lawyers could come after you if you tried to redistribute it. Two, it's likely it will be very difficult to attempt to do that because the code is intentionally obfuscated by a javascript minifier.