Thanks! I hope Doppio can become useful to many more people. :)
There is a proposal for value types [0] that Niko Matsakis [1] and Brendan Eich [2] talked about a couple of years back and could be used to implement 64-bit numbers in JS, but I'm still waiting for an implementation or a more complete proposal. Looks like it might be dead in the water, as the proposal hasn't been edited in quite some time, but I do not have my finger on the pulse of browser standards so I could be wrong.
[0] http://wiki.ecmascript.org/doku.php?id=strawman:value_object...
[1] http://smallcultfollowing.com/babysteps/blog/2014/04/01/valu...