Create a circle out of three points

circle.js

function circleFromThreePoints(p1, p2, p3) {

  var x1 = p1.x;
  var y1 = p1.y;
  var x2 = p2.x;
  var y2 = p2.y;
  var x3 = p3.x;
  var y3 = p3.y;

  var a = x1 * (y2 - y3) - y1 * (x2 - x3) + x2 * y3 - x3 * y2;

  var b = (x1 * x1 + y1 * y1) * (y3 - y2) 
        + (x2 * x2 + y2 * y2) * (y1 - y3)
        + (x3 * x3 + y3 * y3) * (y2 - y1);
 
  var c = (x1 * x1 + y1 * y1) * (x2 - x3) 
        + (x2 * x2 + y2 * y2) * (x3 - x1) 
        + (x3 * x3 + y3 * y3) * (x1 - x2);
 
  var x = -b / (2 * a);
  var y = -c / (2 * a);

  return {
    x: x,
    y: y,
    r: Math.hypot(x - x1, y - y1)
  };
}