Read file:// URLs in IE XMLHttpRequest

How can I get code like this to work?

As suggested above, this looks like a fault in Microsoft XMLHttpRequest. jQuery (Jul 2011) also writes:-

Microsoft failed to properly implement the XMLHttpRequest in IE7 (can't request local files)

I confirm this failure for IE8 too.

A solution is to use new window.ActiveXObject( "Microsoft.XMLHTTP" ) for local files if XMLHttpRequest doesn't work.

The failure is in the xhr.open line so it can be caught there and then try ActiveXObject as follows:-

var xhr = new XMLHttpRequest()
try {
    xhr.open('GET', url, true)
}
catch(e) {
    try {
        xhr = new ActiveXObject('Microsoft.XMLHTTP')
        xhr.open('GET', url, true)
    }
    catch (e1) {
        throw new Error("Exception during GET request: " + e1)
    }
}

This code will at least use standard XMLHttpRequest for IE9 (untested) and future IE browsers if/when Microsoft fixes the fault. With the jQuery code above, non standard Microsoft.XMLHTTP will be used whenever ActiveXObject is available, even if Microsoft fix the fault.

Hmm, could it be the difference between the native XMLHttpRequest object and the ActiveX one? I seem to remember something about that. That is, instead of

var xhr = new XMLHttpRequest();

try

var xhr = new ActiveXObject("MSXML2.XMLHTTP");

Obviously, put some checks in place to see if the browser supports ActiveX. Of course, this is limited to IE only, as well.

I just happened to stumble across exactly the same problem. As suggested above, the non-native ActiveX "constructor" works. I’m not really sure whether there are different policies applied to the two objects, but since jQuery mentions the same problem as well, it may be a genuine bug. Here is the relevant piece of code from the jQuery source (1.4.2, line 4948):

// Create the request object; Microsoft failed to properly
// implement the XMLHttpRequest in IE7 (can't request local files),
// so we use the ActiveXObject when it is available
// This function can be overriden by calling jQuery.ajaxSetup
xhr: window.XMLHttpRequest && (window.location.protocol !== "file:" || !window.ActiveXObject) ?
    function() {
        return new window.XMLHttpRequest();
    } :
    function() {
        try {
            return new window.ActiveXObject("Microsoft.XMLHTTP");
        } catch(e) {}
    }