code: function(point) {
        let x_ = null;
        let y_ = null;
        if (point[0] == extent[0]) {
          x_ = extent[2];
        } else if (point[0] == extent[2]) {
          x_ = extent[0];
        }
        if (point[1] == extent[1]) {
          y_ = extent[3];
        } else if (point[1] == extent[3]) {
          y_ = extent[1];
        }
        if (x_ !== null && y_ !== null) {
          return [x_, y_];
        }
        return null;
      }
docs: find the extent corner opposite the passed corner