Which begs the question: why not use Lua to begin with, at least when starting a project? I totally get the use for this if you've got some existing JS and for some reason you need to talk to C. I've seen crazier things.

But I'd say the solution to 'I need a scripting language to talk to C' is solved very well by Lua.

*Edit:

After looking at the example it's C API is surprisingly similar to Lua's, even using such keywords as userdata. So that's interesting, considering they're really similar languages.

The function js_dofile doesn't look like it's doing anything with lua, nor can I find any use of lua at a cursory look at the source. I'd guess that since Lua and JS share syntax they are loading a .lua file with valid JS syntax.

I noticed that too. My theory is that MuJS started out as a fork of Lua, with all sorts of Ecmascript-compat bolted on. Use with care, I suppose.