Detect Retina Display using Javascript

Firefox 14.0.1
window.retina => undefined
window.devicePixelRatio => undefined

Chrome 21.0.1180.57
window.devicePixelRatio => 2
window.retina => undefined

Safari 6.0 (8536.25)
window.retina => true
window.devicePixelRatio => 2

var retina = (window.retina || window.devicePixelRatio > 1);
if (retina) {
 //retina display
 //load high resolution picture
 $(img).src = 'high-res.jpg';
}
else {
 //non-retina display
 //load low resolution picture
 $(img).src = 'high-res.jpg';
}