Just an implementation of haversine formula in JS

haversine.js

function haversine(lat1,lon1,lat2,lon2)
{
    var R = 6372797.560856;

    var dlat = (lat2-lat1).toRad();
    var dlon = (lon2-lon1).toRad();

	var lonh=Math.sin(dlon*0.5);
	lonh*=lonh;
	
	var lath=Math.sin(dlat*0.5);
	lath*=lath;
	
	var tmp= Math.cos(lat1.toRad())*Math.cos(lat2.toRad());

    return 2*R*Math.asin(Math.sqrt(lath+tmp*lonh));
}