JS has 'real integers', they're not being simulated. If you put the appropriate hints in your JS the JITted output will never use a float anywhere. Your complaint is more that all the operators (with the exception of the bitwise ones) operate on floats, and yes, that is kind of a problem.

64 bit integer support is being worked on for JS elsewhere; asm.js probably doesn't offer it yet since you can't use 64 bit ints in any browser yet.