There is a great deal of misinformation on the web about detecting an iPad or an iPhone in JavaScript. The top answer on stackoverflow - and many blog posts using this technique - are all incorrect.

The conventional wisdom is that iOS devices have a user agent for Safari and a user agent for the UIWebView. This assumption is incorrect as iOS apps can and do customize their user agent. The main offender here is Facebook, whose iOS app alone accounts for about 1-3% of Artsy’s daily traffic.

Compare these user agent strings from iOS devices:

# iOS Safari
iPad: Mozilla/5.0 (iPad; CPU OS 5_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9B176 Safari/7534.48.3
iPhone: Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3

# UIWebView
iPad: Mozilla/5.0 (iPad; CPU OS 5_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Mobile/98176
iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Mobile/8B117

# Facebook UIWebView
iPad: Mozilla/5.0 (iPad; U; CPU iPhone OS 5_1_1 like Mac OS X; en_US) AppleWebKit (KHTML, like Gecko) Mobile [FBAN/FBForIPhone;FBAV/4.1.1;FBBV/4110.0;FBDV/iPad2,1;FBMD/iPad;FBSN/iPhone OS;FBSV/5.1.1;FBSS/1; FBCR/;FBID/tablet;FBLC/en_US;FBSF/1.0]
iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 5_1_1 like Mac OS X; ru_RU) AppleWebKit (KHTML, like Gecko) Mobile [FBAN/FBForIPhone;FBAV/4.1;FBBV/4100.0;FBDV/iPhone3,1;FBMD/iPhone;FBSN/iPhone OS;FBSV/5.1.1;FBSS/2; tablet;FBLC/en_US]

The old way to identify iPhone / iPad in JavaScript:

IS_IPAD = navigator.userAgent.match(/iPad/i) != null;
IS_IPHONE = navigator.userAgent.match(/iPhone/i) != null) || (navigator.userAgent.match(/iPod/i) != null);

If you were to go with this approach for detecting iPhone and iPad, you would end up with IS_IPHONE and IS_IPAD both being true if a user comes from Facebook on an iPad. That could create some odd behavior!

The correct way to identify iPhone / iPad in JavaScript:

IS_IPAD = navigator.userAgent.match(/iPad/i) != null;
IS_IPHONE = (navigator.userAgent.match(/iPhone/i) != null) || (navigator.userAgent.match(/iPod/i) != null);
if (IS_IPAD) {
  IS_IPHONE = false;
}

We simply declare IS_IPHONE to be false on iPads to cover for the bizarre Facebook UIWebView iPad user agent. This is one example of how user agent sniffing is unreliable. The more iOS apps that customize their user agent, the more issues user agent sniffing will have. If you can avoid user agent sniffing (hint: CSS Media Queries), DO IT.