Get the distance between a starting and ending lat/long in JavaScript.

getDistanceBetweenLatLongs.js

;function getDistanceBetweenLatLongs (start, end) {
  // Starting lat, long
	var lat1 = start.lat.toRad();
	var lng1 = start.lng.toRad();

	// Destination lat, long
	var lat2 = end.lat.toRad();
	var lng2 = end.lng.toRad();

	// Earths radius in km
	var rad  = 6371;

	// Degree to radian converter
	Number.prototype.toRad = function() {
	  return this * Math.PI / 180;
	}

	// Shorter namespaces
	var m = Math,
		cos = m.cos,
		sin = m.sin,
		atan2 = m.atan2,
		sqrt = m.sqrt;
			
	// The bearings
	var bear = {
		"lat": (lat2 - lat1).toRad(),
		"lng": (lng2 - lng1).toRad()
	};

	// The point on our plane
	var alpha = sin(bear.lat / 2) * sin(bear.lat / 2) + cos(lat1)
					* cos(lat2) * sin(bear.lng / 2) * sin(bear.lng / 2);

	// The multiplier
  var theta = atan2(sqrt(alpha), sqrt(1-alpha));

  // return the distance
  return rad * theta;
}