For such short distances, and when the accuracy doesn’t have to be exact to the centimeter, you can treat the surface of the earth as flat. Calculate a conversion from degrees to kilometers at the latitude of the center point, then the Pythagorean theorem can be used to get the distance:
function arePointsNear(checkPoint, centerPoint, km) {
var ky = 40000 / 360;
var kx = Math.cos(Math.PI * centerPoint.lat / 180.0) * ky;
var dx = Math.abs(centerPoint.lng - checkPoint.lng) * kx;
var dy = Math.abs(centerPoint.lat - checkPoint.lat) * ky;
return Math.sqrt(dx * dx + dy * dy) <= km;
}
Demo: http://jsfiddle.net/Guffa/57gQa/
Note: The code doesn’t take into consideration if you are passing the 0/360 longitude. If that is the case, you would have to normalize the longitudes first.