CORS with jQuery and XDomainRequest in IE8/9

I've written a proxy which will gracefully downgrade to proxying if IE9 or less is used. You do not have to change your code at all if you are using ASP.NET.

The solution is in two parts. The first one is a jquery script which hooks into the jQuery ajax processing. It will automatically call the webserver if an crossDomain request is made and the browser is IE:

$.ajaxPrefilter(function (options, originalOptions, jqXhr) {
    if (!window.CorsProxyUrl) {
        window.CorsProxyUrl = '/corsproxy/';
    }
    // only proxy those requests
    // that are marked as crossDomain requests.
    if (!options.crossDomain) {
        return;
    }

    if (getIeVersion() && getIeVersion() < 10) {
        var url = options.url;
        options.beforeSend = function (request) {
            request.setRequestHeader("X-CorsProxy-Url", url);
        };
        options.url = window.CorsProxyUrl;
        options.crossDomain = false;
    }
});

In your web server you have to receive the request, get the value from the X-CorsProxy-Url http header and do a HTTP request and finally return the result.

My blog post: http://blog.gauffin.org/2014/04/how-to-use-cors-requests-in-internet-explorer-9-and-below/

POST method is supported, and to make a cross-domain https:// request your calling page would also need to be loaded over https. This is the best article I have found which explains these and other limitations of XDomainRequest in detail:

http://blogs.msdn.com/b/ieinternals/archive/2010/05/13/xdomainrequest-restrictions-limitations-and-workarounds.aspx