Here's the circular radius test that we're doing upon mouse click. hitTest() is called for every entity upon click to determine a positive hit.

circle-click.js

/**
 * @author Kevin Moot
 */

Marker.prototype.hitTest = function(point) {
    return Util.pointInCircle(point, this.positionCenter, this.radiusSquared);
};

Util.pointInCircle = function(point, circleCenter, circleRadiusSquared) {
    // determine vector between center of circle and mouse position
    var subX = circleCenter.x - point.x;
    var subY = circleCenter.y - point.y;

    var distance = Math.pow(subX, 2) + Math.pow(subY, 2);

    return distance <= circleRadiusSquared;
};