JavaScript support levels

Generally, i breakdown JavaScript support into 4 levels. This makes life much easier and as i'm a lazy bas****d it suits me to keep development time down to a minimum.

I'm using something like this...

 window.user = {
 jsSupportLevel : 0,
 init: function(){
  if(!document.getElementById){return;}
  var ua=navigator.userAgent.toLowerCase();
  var platform=navigator.platform.toLowerCase();
  var isPC = (platform.match('win32')||platform.match('win64'))?true:false;
  var isDeskTop = ( ua.match('firefox') || isPC || platform.match('macintel')  )?true:false;
  var support = 1;
  if (window.XMLHttpRequest){support = 2}
  if ( support==2 && isDeskTop ){
   support = 3;
  }
  this.jsSupportLevel =  support;
 }
};user.init();

This will give you a js global "user" and you use it in your js to determine whether to do some enhancements or not.
e.g.

if (user.jsSupportLevel>1){
   ... attach Ajax related event handlers ...
} 

Example support levels

0 = user-agent doesn't understand DOM (!document.getElementById)
1 = user-agent DOM enabled
2 = user-agent has native ajax support (window.XMLHttpRequest)
3 = user-agent is probably a desktop browser

You can use the above to deliver the appropriate level of JavaScript support to users, e.g. Don't implement any JavaScript if the support level is 0, only do very basic JavaScript for level 1, do pretty much everything for level 2 and level 3 is level 2 with transitions and opacity (stuff you wouldn't want on a mobile/hand-held).

This will only work if your using progressive enhancement of course!