Question

I need to do integer division in JavaScript, which only gives me double-precision floating point numbers to work with. Normally I would just do Math.floor(a / b) (or a / b | 0) and be done with it, but in this case I’m doing simulation executed in lockstep and need to ensure consistency across machines and runtimes regardless of whether they use 64-bit or 80-bit internal precision.

I haven’t noticed any inconsistencies so far, but I haven’t been able to conclusively convince myself that they can’t happen. So I’m left wondering:

1. Assuming a and b are integers in range 0..2^31-1 and 1..2^31-1 respectively, are results from JavaScript Math.floor(a / b) (and a / b | 0) guaranteed to be consistent across machines and runtimes?

2. Why or why not?

Answer 1

My guess would be no. And the answer would be that it’s dependent upon a number of factors:

1. Browser vendor implementations of ECMA Script.

2. Whether or not a particular version of ECMA Script specifies that level of consistency (typically not).

3. Other, external factors on the end-users’ machines that you may not be aware of.

Floating point arithmetic is notoriously susceptible to rounding errors. While it’s nice to think it’s accurate with all those digits to the right-hand side of the decimal point, getting two machines running vastly different hardware and software configurations to agree on a calculation can be like herding cats.