This is not the most power efficient setup (as I said, there are far, far more efficient cards, especially when run in tandem) but it is the most sought-after card due to density. Regardless, it's exceptionally profitable to mine if you already own the card.

JS mining only takes a slight performance hit compared to native, actually; asm.js and others provide very good results since the problem is memory-hard, not computationally bound. It is definitely energy efficient and worth doing if all you care about is ROI vs. marginal utility rates in most American cities (perhaps not some of California). Nicehash built their business model around exactly that, which is the real reason people like libraries like the OP's - it's a distributed way to get people mining, not for the individual person themselves.