140byt.es Cross-browser XHR

annotated.js

/* arguments are defined but ignored */
function(s, a){
	/*
	Here:
	a = "Msxml2.XMLHTTP"
	((anonymous array)) = ["Msxml2.XMLHTTP"]
	((anonymous array)) = ["Msxml2.XMLHTTP", "Msxml2.XMLHTTP.3.0"]
	((anonymous array)) = ["Msxml2.XMLHTTP", "Msxml2.XMLHTTP.3.0", "Msxml2.XMLHTTP.6.0"]
	a = ((that anonymous array))
	*/
	a = [a = "Msxml2.XMLHTTP", a + ".3.0", a + ".6.0"];

	do /* try..catch do not requires {} */
		try {
			/*
			Get the last element, so during the loop s variable is:
			
			"Msxml2.XMLHTTP.6.0"
			"Msxml2.XMLHTTP.3.0"
			"Msxml2.XMLHTTP"
			undefined
			*/
			s=a.pop();
			
			
			/* try to return new... */ return new  /* ... XHR if s==undefined (last iteration), AX if not */ (s ? ActiveXObject : XMLHttpRequest) /*... using s itself as an argument */(s)
		} catch(e){
			/* if it is failed do nothing and leave ; for very old version of Firefox */;
		}
	/* if s == undefined (and a.length == 0) that was a last iteration and exit */
	while(s)
	/* thats it, return undefined if all previous return attempts failed */
}

index.js

function(s,a){a=[a="Msxml2.XMLHTTP",a+".3.0",a+".6.0"];do try{s=a.pop();return new(s?ActiveXObject:XMLHttpRequest)(s)}catch(e){;}while(s)}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 YOUR_NAME_HERE <YOUR_URL_HERE>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "crossbrowser_XHR",
  "description": "Function creates XMLHTTPRequest object in IE6+ and other browsers.",
  "keywords": [
    "xhr"
  ]
}

The use of various ActiveX progIDs is pretty misunderstood.
You can check out this rad 3 page thread on the topic (read through all pages, it's a bit old but does the job).
To be honest I don't know the benefit of using 6.0 over 3.0 vs plain MSXML2.XMLHTTP, the post does explain some possible differences between MSXML2.XMLHTTPand Microsoft.XMLHTTP though.

In practice jQuery seems to be doing fine with just using Microsoft.XMLHTTP.