docs: function getAreaInternal(coordinates, radius) {
    let area = 0;
    const len = coordinates.length;
    let x1 = coordinates[len - 1][0];
    let y1 = coordinates[len - 1][1];
    for (let i = 0; i < len; i++) {
      const x2 = coordinates[i][0];
      const y2 = coordinates[i][1];
      area += toRadians(x2 - x1) *
          (2 + Math.sin(toRadians(y1)) +
          Math.sin(toRadians(y2)));
      x1 = x2;
      y1 = y2;
    }
    return area * radius * radius / 2.0;
  }