A simple class to allow 64-bit calculations in Javascript. Not a polyfill.
More info: https://medium.com/@nxtchg/simplifying-int64-polyfill-4a9c35441e25
var v = new Int64(123);
v.add(4).mul(555);
var js_num = v.toNumber();
See the code for all available methods, it's just 260 lines.