Limit concurrent jQuery ajax requests to at most 3 at a time, and queue the rest.

AjaxQueue.js

var ajaxReqs = 0;
var ajaxQueue = [];
var ajaxActive = 0;
var ajaxMaxConc = 3;
function addAjax(obj) {
    ajaxReqs++;
    var oldSuccess = obj.success;
    var oldError = obj.error;
    var callback = function() {
        ajaxReqs--;
        if (ajaxActive === ajaxMaxConc) {
            $.ajax(ajaxQueue.shift());
        } else {
            ajaxActive--;
        }
    }
    obj.success = function(resp, xhr, status) {
        callback();
        if (oldSuccess) oldSuccess(resp, xhr, status);
    };
    obj.error = function(xhr, status, error) {
        callback();
        if (oldError) oldError(xhr, status, error);
    };
    if (ajaxActive === ajaxMaxConc) {
        ajaxQueue.push(obj);
    } else {
        ajaxActive++;
        $.ajax(obj);
    }
}

• The mistake in the original code is that it doesn't check if ajaxQueue still contains elements. If at some point ajaxQueue gets empty, the code ends up with a (harmless) no-op $.ajax(undefined), and importantly, ajaxActive isn't decremented, which leads to various issues.
• Your fixed code has a mistake, there is an extraneous ajaxActive--;. You may either:
  • Remove the "else" branch having the ajaxActive--;
  • Remove the first ajaxActive--;, and the ajaxActive < ajaxMaxConc and the ajaxActive++; (when callback() triggers a new request, the number of active requests stays the same)
• ajaxActive cannot exceed ajaxMaxConc, and this is ensured when addAjax() is called. Therefore it is irrelevant to check it in callback().
• Btw, notice the ajaxReqs variable is useless.

Considering the above points, I ended up with the following code for callback():

    var callback = function () {
        if (ajaxQueue.length > 0) {
            $.ajax(ajaxQueue.shift());
        } else {
            ajaxActive--;
        }
    }