LatLonToMercator.js

function LatLonToMercator(lat, lon) {
 
    var rMajor = 6378137; //Equatorial Radius, WGS84
    var shift  = Math.PI * rMajor;
    var x      = lon * shift / 180;
    var y      = Math.log(Math.tan((90 + lat) * Math.PI / 360)) / (Math.PI / 180);
    y = y * shift / 180;
 
    return {'X': x, 'Y': y};
}

//example use
var XY = LatLonToMercator(38.878586, -76.989626);
alert('Mercator X:' + XY.X + ', Mercator Y:' + XY.Y);