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"
  ]
}

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

My solution with saving 3 bytes.

interesting... are you sure this won't interfere with normal operation of XMLHttpRequest?

I am not sure but I think it won't because XMLHttpRequest.length returns 0 (just ignore the arguments object). Actually it seems working well.

is the this needed for some reason?

It is needed. If the XMLHttpRequest doesn't exist in the global object, the browsers will throw an error, but this.XMLHttpRequest(the this here is actually window) will be undefined instead and go to ActiveXObject.

awesome, i've reflected the change. thanks @tsaniel!

Just for the records, is there any IE version without a native XMLHttpRequest and support for Microsoft.XMLHTTP?

It's quite confusing indeed. According to http://msdn.microsoft.com/en-us/library/ms537505(v=vs.85).aspx#_id , I think we can simply use new ActiveXObject('MSXML2.XMLHTTP.3.0').

yikes, really? since this code was ported from PPK, i'm inclined to think it's good enough for now.

So just keep the code unchanged... until someone finds another better solution.

So instead of the current function, we could simply use

(function(){try{return new(this.XMLHttpRequest||ActiveXObject)('MSXML2.XMLHTTP.3.0')}catch(e){}}

@atk It should be:

(function(){try{return new(this.XMLHttpRequest||ActiveXObject)('MSXML2.XMLHTTP.3.0')}catch(e){}}());

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