Javascript is popular outside the browser not because it's a great programming language, but because it gets us back to where we were before the browser became the dominant app delivery platform: it's now again possible to write your entire client-server app in one language.
If you're talking Node.js, sure, but that was a stepping stone. It's now being used for all sorts of weird things, like WinRT, and as GNOME's flagship desktop app language. These need not have any server-side components.
A large part is obviously that JavaScript is now well known and many developers have been exposed to it, but also because a minimal prototype language with first class functions is often all you need or want, just like Lua. (I assume this has been realized after exposure to JavaScript, or maybe I'm giving JS devs too much credit and they're not fans of minimal and clean languages.)
