var activeAJAX = 0;

在进行 AJAX 调用之前，activeAJAX++;

Before making an AJAX call, activeAJAX++;

完成 AJAX 调用后(在回调中): if (--activeAJAX == 0) { allDone();}

After completing an AJAX call (in the callback): if (--activeAJAX == 0) { allDone(); }