Haversine formula for stable distance calculation between pairs of coordinates.

haversine.js

// All angles in radians.
function haversine(aLong, aLat, bLong, bLat) {
	var R = 6371; // Radius of Earth.
	
	var latDiff = bLat - aLat;
	var longDiff = bLong - aLong;

	var step1 = Math.sin(latDiff / 2);
	var step2 = Math.sin(longDiff / 2);
	var step3 = step1 * step1 + step2 * step2 * Math.cos(aLat) * Math.cos(bLat);
	var step4 = 2 * Math.atan2(Math.sqrt(step3), Math.sqrt(1 - step3));

	return R * step4;
}