Wait until another thread is working in jQuery

Question

Here, I am waiting for html button element from the popup iframe window. When a popup is open then my block of code will fire the click event on time and it's working fine, but my query is wanting to wait until not get button.

 

I have used the nested function calling with timeout thread, so when I call then work, but don't wait and go to the next task.

setPrint();
function setPrint() {
   setTimeout(function () {
      var btnprint = null;
      var frame = $('#WindowFadeiFrame').get(0).contentDocument;
      var btn = $('#btn', frame);
      if (btn.length == 0) {
         btnprint == null
         console.log('not found');
      }
      else {
         btn.click();
         btnprint = btn;
         console.log('found now');
      }
      if (btnprint == null) {
         setPrint(); }
   }, 100);
}