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. Source jsCuckoo.js
Scripts for copy-paste or include in other projects int64.js

Other oldie:
For serious BigNumber math try this site: http://jsfromhell.com/classes/bignumber