Returns the difference of two angles given in radians. The result is always between -PI and PI.

angleDifference.js

// Returns the difference of two angles given in radians. 
// The result is always between -PI and PI.

function angleDifference(angle1, angle2) {
	const TWO_PI = Math.PI * 2;
  let a1 = (angle1 % TWO_PI + TWO_PI) % TWO_PI;
  let a2 = (angle2 % TWO_PI + TWO_PI) % TWO_PI;

  if (a2 > a1) {
    let d1 = a2 - a1;
    let d2 = a1 + TWO_PI - a2;
    if (d1 <= d2) {
      return -d1;
    } 
    else {
      return d2;
    }
  } 
  else {
    let d1 = a1 - a2;
    let d2 = a2 + TWO_PI - a1;
    if (d1 <= d2) {
      return d1;
    } 
    else {
      return -d2;
    }
  }
}