Computes the area of a geographic bounding box assuming spherical Earth

bounding-box-area.js

// adapted from http://mathforum.org/library/drmath/view/63767.html

const r = 6371.0072; //earth radius in KM

function toRadians(degrees) {
  return degrees * Math.PI / 180;
}; 

function computeArea(bbox){
  const [minLng, minLat, maxLng, maxLat] = bbox;
  const h = r * ( Math.sin(toRadians(maxLat)) - Math.sin(toRadians(minLat)))
  const latitudinalArea = 2 * Math.PI * r * h; 
  const longitudinalSection = (maxLng - minLng) / 360; 
  return latitudinalArea * longitudinalSection;
}

//example  [west, south, east, north]
const colorado = [-109.060256958008, 36.9924240112305, -102.041580200195, 41.0023612976074];
computeArea(colorado); //returns 270391.9194161927 square kilometers