Calculate shortest distance between two points

calculateDistance.js

function calculateDistance(lat1, lon1, lat2, lon2) {
	var R = 6371; // km
	var dLat = (lat2 - lat1).toRad();
	var dLon = (lon2 - lon1).toRad();
	var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
		Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) *
		Math.sin(dLon / 2) * Math.sin(dLon / 2);
	var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
	var d = R * c;
	return d;
}
Number.prototype.toRad = function() {
	return this * Math.PI / 180;
}