I’ve developed a function to calculate the exit angle of a projectile after it collides with a boundary/edge inside a rectangular space, based on the entry angle and the axis it is colliding with.

Given those two factors, the actual boundary is implied, since a projectile travelling at an angle of 315° (where 0°/360° sit at 3 o’clock) and colliding with a horizontal boundary (the x axis) must be colliding with the top boundary.

The current implementation came about through iterative refactoring of a much more verbose and redundant version, however I can’t vocalise exactly why some parts are necessary, but they appear to satisfy the requirements and I’m confident it works based on a number of tests.

function reflectAngle(angle, axis) {      if (angle % 90 === 0)         return (angle + 180) % 360;      let reflected;     let segment = (angle % 90) * 2;      if (axis === 'X')         reflected = angle + (180 - segment);      else if (axis === 'Y')         reflected = angle - segment;      if (angle < 90)         reflected += 180;      return reflected % 360; } 

Can this function be simplified or improved for readability?

^{It’s a fairly language agnostic function so I’m not heavily interested in JS-specific improvements, but happy to hear them.}