An unsigned int64 type variable here is an object with two 32-bit properties
.h - highest 32-bits (register AH)
.l - lowest 32-bits (register AL)

All math is for unsigned case only.

Samples:

It is not good for operations as mul,div,pow,sqrt,...
Not a case of fast performance.
Anyway, properly working sample at site: jsCuckoo chess.
Scripts for copy-paste are here

More advanced library: http://www.pps.jussieu.fr/~canou/Int64.js
For serious BigNumber math try this site: http://jsfromhell.com/classes/bignumber
or http://www.leemon.com/crypto/BigInt.html
Another BigInt.js.