Join 2 'threads' in javascript

Question

If I have an ajax call off fetching (with a callback) and then some other code running in the meantime. How can I have a third function that will be called when both of the

Answer 1

Something like this (schematic):

registerThread() {
  counter++;
}

unregisterThread() {
  if (--counter == 0) fireEvent('some_user_event');
}

eventHandler_for_some_user_event() {
   do_stuff();
}