MercatorToLatLon.js

function MercatorToLatLon(mercX, mercY) {
 
    var rMajor = 6378137; //Equatorial Radius, WGS84
    var shift  = Math.PI * rMajor;
    var lon    = mercX / shift * 180.0;
    var lat    = mercY / shift * 180.0;
    lat = 180 / Math.PI * (2 * Math.atan(Math.exp(lat * Math.PI / 180.0)) - Math.PI / 2.0);
 
    return { 'Lon': lon, 'Lat': lat };
}

//example use
var LatLon = MercatorToLatLon(-10072092.937600996, 5100841.6350086592);
alert('Result: Lat = ' + LatLon.Lat + ', Lon = ' + LatLon.Lon);