JavaScript: Position rotation

gistfile1.js

function getRotatedPosition(x, y, rotation, referenceX, referenceY) {
    if (isNaN(referenceX)) referenceX = 0;
    if (isNaN(referenceY)) referenceY = 0;

    var radian = rotation * (Math.PI / 180);

    return {
        x:x * Math.cos(radian) - y * Math.sin(radian) + referenceX - referenceX * Math.cos(radian) + referenceY * Math.sin(radian),
        y:x * Math.sin(radian) + y * Math.cos(radian) + referenceY - referenceX * Math.sin(radian) - referenceY * Math.cos(radian)
    };
}