Really? I've never seen it used as an actual extension language. I've seen Lua, I've seen Scheme, at work we do everything in Python (but that's more extending than embedding), but I've never seen JavaScript as a "extend this C or C++ application" language.
I'm not counting node, because it only exists to run JavaScript. Nor am I counting web browsers, for similar reasons.
Can you give some examples? PDF is the only one I can think of. Most of the existing JavaScript engines seem too heavy-weight to be used as extensions.
Since I'm not in the web-developer bubble, I just don't see JavaScript all that often.