get a supported XHR instance

annotated.js

function(
  a // cursor placeholder
){
  for(                    // for all a
    a = 3;                // from 3
    a--;                  // to 0,
  ) try {                 // try
    return new(           // returning a new
      this.XMLHttpRequest // XMLHttpRequest (w3c)
      ||                  // or
      ActiveXObject       // ActiveXObject (MS)
    )([                   // reflecting
      "Msxml2",           // the
      "Msxml3",           // various
      "Microsoft"][a] +   // MS flavors
      ".XMLHTTP"          // and appropriate suffix,
    )
  }
  
  catch(e){}              // and ignore when it fails.
}

index.js

function(a){for(a=3;a--;)try{return new(this.XMLHttpRequest||ActiveXObject)(["Msxml2","Msxml3","Microsoft"][a]+".XMLHTTP")}catch(e){}}

LICENSE.txt

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

 Copyright (C) 2011 Jed Schmidt <http://jed.is>

 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": "getXHR",

  "description": "get a supported XHR instance",

  "keywords": [
    "ajax",
    "XHR",
    "cross-browser",
    "XmlHttpRequest"
  ]
}

I took the liberty to extend the code just a bit more (even if it isn't 140bytes anymore).