javascript distance between two coordinates

calculateDistance

function calculateDistance(lat1, lon1, lat2, lon2) {
    var R = 6371;
    var dLat = (lat2 - lat1) * Math.PI / 180;
    var dLon = (lon2 - lon1) * Math.PI / 180;
    var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
            Math.cos(lat1 * Math.PI / 180) * Math.cos(lat2 * Math.PI / 180) *
            Math.sin(dLon / 2) * Math.sin(dLon / 2);
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    var d = R * c;
    if (d > 1)
        return Math.round(d) + "km";
    else if (d <= 1)
        return Math.round(d * 1000) + "m";
    return d;
}