A variant of atan.
Converts both arguments to doubles.
Returns the angle in radians between the positive x-axis
and the vector (b,a).
The result is in the range -PI..PI.
If b is positive, this is the same as atan(b/a).
The result is negative when a is negative (including when a is the
double -0.0).
If a is equal to zero, the vector (b,a) is considered parallel to
the x-axis, even if b is also equal to zero. The sign of b determines
the direction of the vector along the x-axis.
Returns NaN if either argument is NaN.
Source
external double atan2(num a, num b);