Minimal ajax function. No fallback for older browsers (lt IE7).

ajax.js

;(function(exports){
	exports.ajax = function(opt){
		var req = new XMLHttpRequest(),
			res,
			opt = opt || {};

			//defaults
			opt.success	= opt.success || function(){};
			opt.params	= opt.params || {};
			opt.type	= opt.type || 'GET';
			opt.contentType = opt.contentType || "application/x-www-form-urlencoded";

			//default events
			req.onerror	= opt.failure;
			req.onabort	= opt.abort;
			req.onload	= opt.load;

			opt._params = "";
			for(p in opt.params){
				opt._params += p +"="+ encodeURI(opt.params[p]) +"&";
			}

			req.onreadystatechange = function(){
				if(req.readyState < 4) return;
				if(req.status < 200 && req.status > 206) return;
				try {
					res = JSON.parse(req.responseText);
				}catch(e){
					res = req.responseText;
				}
				opt.success(res, req);
			}

			if(opt.type == 'GET'){
				req.open(opt.type, opt.url+"?"+opt._params, true);
				req.send(null);
			}else{
				// default alternative is POST
				req.open(opt.type, opt.url, true);
				req.setRequestHeader("Content-type", opt.contentType);
				req.send(opt._params);
			}
	}

})(window);