Have a quadratic bezier curve, need to figure out an angle along the way. This should work quite well, easy to convert to JavaScript.

Calculate Angle Along Quadratic Bezier

public static function quadraticBezierAngle(value:Number, anchor1:Point, anchor2:Point, control:Point):Number {
    var uc:Number = 1 - value;
    var dx:Number = (uc * control.x + value * anchor2.x) - (uc * anchor1.x + value * control.x);
    var dy:Number = (uc * control.y + value * anchor2.y) - (uc * anchor1.y + value * control.y);
    return Math.atan2(dy, dx);
}

If anyone wants the JavaScript version:

function quadraticBezierAngle(t, anchor1, anchor2, control) {
  let u = (1 - t);
  let dx = (u * control.x + t * anchor2.x) - (u * anchor1.x + t * control.x);
  let dy = (u * control.y + t * anchor2.y) - (u * anchor1.y + t * control.y);

  return Math.atan2(dy, dx);
}