I'm seeing differences between Firefox and Safari in the last digit of the output from Math#atan2.
My code:
Math.atan2(-0.49999999999999994, 0.8660254037844387)
Safari (12.1.1) gives -0.5235987755982988 but Firefox (Mac/67.0) gives -0.5235987755982987.
This is of course a tiny difference. However, it seems that all implementations should yield the same output across all inputs. A difference like this could, for example, cause an if statement to follow different paths depending on browser.
Does what I'm seeing violate any version of the ECMAScript spec?
The ECMAScript 2015 spec has this to say:
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log,log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms.The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2008 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
The 5.1 spec has similar language.
So I think it's safe to say this behavior doesn't violate the spec.