code: function getSqSegDist(p, p1, p2) {
  
      var x = p1.x,
          y = p1.y,
          dx = p2.x - x,
          dy = p2.y - y;
  
      if (dx !== 0 || dy !== 0) {
  
          var t = ((p.x - x) * dx + (p.y - y) * dy) / (dx * dx + dy * dy);
  
          if (t > 1) {
              x = p2.x;
              y = p2.y;
  
          } else if (t > 0) {
              x += dx * t;
              y += dy * t;
          }
      }
  
      dx = p.x - x;
      dy = p.y - y;
  
      return dx * dx + dy * dy;
  }
docs: square distance from a point to a segment