src/planar.js

export function planarTriangleArea(triangle) {
  var a = triangle[0], b = triangle[1], c = triangle[2];
  return Math.abs((a[0] - c[0]) * (b[1] - a[1]) - (a[0] - b[0]) * (c[1] - a[1])) / 2;
}

export function planarRingArea(ring) {
  var i = -1, n = ring.length, a, b = ring[n - 1], area = 0;
  while (++i < n) a = b, b = ring[i], area += a[0] * b[1] - a[1] * b[0];
  return Math.abs(area) / 2;
}