Mapping clock angle using Math.atan2

Mapping a clock angle onto screen.js

/**
 * 1 - Start off everything from the origin (0, 0) just like how to draw something on a graph in Mathematics 
 *     where the Cartesian Plane has a +ve and -ve x- and y- axes and the circle is drawn on it by having a 
 *     centroid at the origin (0, 0);
 * 2 - Do the same thing for finding an angle from a coordinate as you know a circle has 360 degree, however 
 *     the method to find a particular angle from the coordiante you have chosen is kind of not doing what 
 *     you would have expected. Say, the method you use to find the angle is by using Math.atan2, it actually 
 *     returns a computed angle from the X axis and the range of the returned value falls between -pi and +pi. 
 */
 /**
  * 3 - Conversion is needed for the returned value to make them fit into the degree of a circle which has 360
  *     instead of any angle of -ve value. To do so, see the code below:
 */ 
 
// Regular method to find an angle from a coord. on a Cartesian plane.
  function _evalAngleFromCircleAtOrigin(point_from_y_axis, point_from_x_axis) {
    return Math.atan2(point_from_y_axis, point_from_from_x_axis);
  }
  
// Method to find an angle from a given coord. on a computer graphics.
  function _evalAngleFromCircleAtOriginWithConversion(point_from_y_axis, point_from_x_axis) {
    /**
     * - invert x and y due to compute graphics has only +ve and -ve x- and y-axes so that the angle will 
     *   be computed from the y axis instead of x axis.
     * - turning y into -ve value is to invert the values of y so that it starts from the top of inverted y-axis.
     */
    return Math.atan2(point_from_x_axis, -(point_from_y-axis));
  }
  
// Further convert the return angle to any value from 0 to 2pi.
  function _convertAngle(angle) {
    /**
     * - the above method only transform how normally the atan2 method finds an angle from a circle on the 
     *   Cartesian plane and by doing that everything will look exactly how a typical clock works, having the number
     *   12 from the top of the clock and moving indefinitely in clockwise direction.
     * - however, the second half of the circle (or clock) are still in -ve values and we need to do some magic to 
     *   make those numbers to become +ve so that the degree will go from 0 to 360 in the whole circle in each 
     *   successive clockwise direction.
     */
    return angle <= 0: 360 + angle: angle;
  }