Calculage distance between 2 points on Google Maps

distance_2_poins.js

function distance(lat1,lon1,lat2,lon2) {
    var R = 6371; // km (change this constant to get miles)
    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;
}