how to convert decimal degrees to miles + Javascript

To convert the decimal degrees to miles :

function distanceConverter(x1,y1,x2,y2)
{
// distance between 2 points from decimal degrees to miles
// where x values represent longitude
// where y values represent latitiude
dx = 69.172 * (x2 - x1) * Math.cos( y1 / 57.3);
dy = 69.172 * (y2 - y1);
var dist = Math.sqrt(dx * dx + dy * dy);
return dist;
}