Detect the specific iPhone/iPod touch model in Javascript

I am developing a website for Apple i* devices which uses HTML5 webkit features like transitions but I want to disable some of the fancy stuff for older/slower iPhone models like iPhone < 3GS and iPod touch < 3rd Gen because on those devices the transitions are too slow.

Is there a way to detect the exact model (not just the O开发者_Python百科S/User Agent) within Javascript?

This appears to work, at least to determine between 2 and 3g and 3gs... Don't know how to figure out if it is a 4g yet...

var percentmobile_t = new Date().getTime();
var percentmobile_s = 0;
while(new Date().getTime() - percentmobile_t < 20)
{
    Math.random();
    percentmobile_s++;
}
_is2_or_3g = (percentmobile_s < 1000);