jQuery override to Ajax method to support queuing requests. A deferred object is returned to act as a proxy when the request is actually sent. By default, GET requests are not queued unless the `queue` option is `true`.

jquery-ajax-queue.js

(function(root, factory) {
    if (typeof define == 'function' && define.amd) {
        define(['jquery'], function(jQuery) {
            factory(jQuery);
        });
    } else {
        factory(root.jQuery);
    }
})(this, function(jQuery) {

    var ajax = jQuery.ajax,
        requestQueue = [],
        requestPending = false;

    function sendRequest(options, promise, trigger) {
        var complete, error, options, params, success, that = this;

        if (trigger !== false) trigger = true;
        if (trigger) requestPending = true;

        success = options.success;
        error = options.error;
        complete = options.complete;

        params = {
            complete: function(xhr, status) {
                if (complete) complete.apply(that, arguments);
                if (trigger) dequeueRequest();
            },
            success: function() {
                if (success) success.apply(that, arguments);
                promise.resolveWith(this, arguments);
            },
            error: function() {
                if (error) error.apply(this, arguments);
                promise.rejectWith(this, arguments);
            }
        };

        ajax(jQuery.extend({}, options, params));
    };

    function dequeueRequest() {
        var args, options, promise;
        if ((args = requestQueue.shift())) {
            options = args[0], promise = args[1];
            sendRequest(options, promise);
        } else {
            requestPending = false;
        }
    };

    function queueRequest(options) {
        var promise, type, queue;
        promise = $.Deferred();
        type = (options.type || 'get').toLowerCase();
        queue = options.queue != null ? options.queue : (type === 'get' ? false : true)

        if (queue && requestPending) {
            requestQueue.push([options, promise]);
        } else {
            sendRequest(options, promise, queue);
        }
        return promise;
    };

    jQuery.ajax = function(options) {
        return queueRequest(options);
    };

    jQuery.hasPendingRequest = function() {
        return requestPending;
    };
    
});