Subdomain tunneling with jQuery and document.domain

subdomain.js

/**
 * Replace $.ajax on your subdomain with a copy taken
 * from your base domain. All jQuery AJAX actions go
 * through $.ajax (i.e. $.get, $.post), so it's all good.
 */
(function() {
    var iframe,
        onload,
        queue = [],

    // This has to be set both here and in iframe.html
    document.domain = 'example.com';
    
    // Back up this page's copy of $.ajax
    window.$._ajax = window.$.ajax;

    // We'll first replace $.ajax with a thin wrapper that both
    // loads our iframe tunnel and saves invocations until the
    // iframe is ready

    $.ajax = function(params) {

        if (!iframe) {

            // tunnel.html should be a bare bones html page that
            // includes a copy of jQuery, and sets document.domain
            // to 'example.com'
            
            iframe = $('<iframe>')
                .attr('src', 'http://example.com/tunnel.html')
                .load(onload)
                .appendTo('head')[0];
        }

        // Save calls to $.ajax, execute when we're ready
        queue.push(params);
    };

    function onload() {
    
        // Our prize: a version of $.ajax that can communicate safely
        // with our base domain
        window.$.ajax = iframe.contentWindow.jQuery.ajax;

        // Flush queued $.ajax calls
        $.each(queue, function(_, params) {
            $.ajax(params);
        });

        queue = null;
    }
})();

I used this solution probably about 12 years ago as i had to support IE6. This now wont work due to all the major browsers slowly making document.domain immutable. https://developer.chrome.com/blog/document-domain-setter-deprecation/ . Working on an alternative.