if ( !window.XMLHttpRequest )
XMLHttpRequest = function(){
return new ActiveXObject("Microsoft.XMLHTTP");
};

Um… I’ve never had Firefox/Firebug generate an error using Prototype, nor accessing its Ajax object. Not once.

new function(){return 1} -> [object Object]

5 * (new function(){return 1}) -> NaN

If it wasn’t the XMLHttpRequest, or “else” clause would’ve been written, as I mentioned in my code – the two browsers would present different results.

You can put a bit more characters and make it work as it should be and still maintaining the ‘good programming’, by asking for ‘window.XMLHttpRequest’ and assigning to it.


if (!window.XMLHttpRequest) {
window.XMLHttpRequest=function () {
return new ActiveXObject('Microsoft.XMLHTTP');
}
}


What I wanted to introduce, after the if then else checking browser version stuff and the magic try/catch Prototype solution is a new approach that would consist of defining once for all (at run time) a function named XMLHttpRequest in order not to introduce a new name for instanciating AJAX calls (when not using any framework of course).
Feel free to adapt the idea to your needs (several ActiveXObject uses, etc..)

//————————————————————–
// constructor
//————————————————————–
this.setGlobals = function(){
if(window.ActiveXObject){isIE = true;}
objREQ = isIE == false ? new XMLHttpRequest() : new ActiveXObject(“Microsoft.XMLHTTP”) ;
my_AJAX = this;
}

On IE6, !XMLHttpRequest doesn’t return false here.
That’s because IE reads the “function”s on the page before running thru the code. It means, it reads the code, finds all the functions and makes the

functions list for the current scope. When it gets to “function XMLHttpRequest” it defines window["XMLHttpRequest"]=function () { … } (window is the

current scope). Only THEN, after declaring the functions, it goes thru the whole code from line 1.

This is NOT true for Gecko.

function a() {
alert("a");
}
if (false) function a() {
alert("b");
}
a();

In the above code, IE will shout “b”, and FF will shout “a”. That is because what I explained before.
Accessing to a() is available before “function a()” is declared, and IE takes the LAST declaration no matter where it is (assuming it’s in the same scope).
FF declares the if statement as scoped statement.
The following code will explain:

a();

if (true) {
function a() {
alert("a");
}
a();
}
function a() {
alert("b");
}

For FF, the result is “b”, “a”.
For IE6, it’s “b”, “b”.

On IE then, the !XMLHttpRequest returns TRUE because it is declared.
FF ignores the statement because XMLHttpRequest does exist and as mentioned before, doesn’t declares “function XMLHttpRequest” as a functions.

BTW, declaring “f=function ()” is different than “function f()”. The latter is accessible from everywhere in the same scope, unlike the first that accessible only after being declared.

why is this news? the try/catch thing used by most frameworks is because Microsoft.XMLHTTP is not always available (depends on the version of MSXML, or something).

by removing the other versions, you will be restricting your IE audience.