This is a pretty lightweight way of detecting when your javascript is living in a mobile device.

if(typeof window.orientation != 'undefined') { /* put mobile spesific code here */ }

To omit spesific devices from running this code you can also add individual checks like the following.

if(typeof window.orientation != 'undefined' && !navigator.userAgent.match(/iPad/i)) {
  /* put mobile spesific(but not ipad) code here */
}

This is a mix of the answers found on stack overflow.