lua's also got an excellently designed embedding API and FFI from Lua to the embedding host that makes JSC look baroque and SpiderMonkey look horrendous: in fact it's the first embedding API I've seen in years that I'd call pleasant to use. It was designed for embedding, and it shows.
Its only real problem is that some of its more recent features (e.g. modules) are only really documented in a dead-tree book (there's a publically-viewable reference manual, and a user guide for the previous version of lua, but a lot of Lua's power lies in nifty ways of using things, and *that* is only in the dead-tree book so far). Even here it beats JS, though, as all you have with JS is the standard, which is appallingly unreadable even by standards' standards, while the publically-available docs for lua are very well-written (as is the dead-tree book: recommended).