It seems to me it is not a "real" AI. I began a game, and after complete development on my side (about 5-6 moves), the opponent AI did only developed pawns, which shows off its limits.

I am not a javascript expert. but given the size of the code, I doubt there is a great IA underluying. Actually I've browsed the mignified code in order to find the AI implementation. The only clue I spot was a call to the Random() function. I won't state it for sure, because I wasn't able to understand the whole code. But I highly guessed it is the basis of the AI implementation. :-)

One can write a decent negamax AI in < 10 lines of code.